Dinosaur meaning of word

Dinosaurs Temporal range: Late Triassic–Present, 233.23 – 0 Mya (Range includes birds (Aves)) PreꞒ Ꞓ O S D C P T J K Pg N (Possible Middle Triassic record)
A compilation of dinosaur skeletons. Clockwise from top left: Stegosaurus stenops (a plated stegosaur), Herrerasaurus ischigualastensis (a carnivorous basal dinosaur), Apatosaurus louisae (a giant sauropod), Edmontosaurus regalis (a duck-billed ornithopod), Microraptor gui (a winged theropod), Triceratops horridus (a horned ceratopsian)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sauropsida
Clade:	Archosauria
Clade:	Avemetatarsalia
Clade:	Ornithodira
Clade:	Dinosauromorpha
Clade:	Dinosauriformes
Clade:	Dracohors
Clade:	Dinosauria Owen, 1842
Major groups
†Ornithischia †Sauropodomorpha Theropoda Aves Possible dinosaurs of uncertain affinity †Alwalkeria? †Chilesaurus †Chindesaurus? †Daemonosaurus? †Eodromaeus? †Nhandumirim †Nyasasaurus? †Pisanosaurus? †Smok? †Tawa? †Thecospondylus †Guaibasauridae? †Herrerasauria?[1][2] †Silesauridae? (paraphyletic?)[3][4][5][6]

Dinosaurs are a diverse group of reptiles^{[note 1]} of the clade Dinosauria. They first appeared during the Triassic period, between 245 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is a subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 mya and their dominance continued throughout the Jurassic and Cretaceous periods. The fossil record shows that birds are feathered dinosaurs, having evolved from earlier theropods during the Late Jurassic epoch, and are the only dinosaur lineage known to have survived the Cretaceous–Paleogene extinction event approximately 66 mya. Dinosaurs can therefore be divided into avian dinosaurs—birds—and the extinct non-avian dinosaurs, which are all dinosaurs other than birds.

Dinosaurs are varied from taxonomic, morphological and ecological standpoints. Birds, at over 10,700 living species, are among the most diverse groups of vertebrates. Using fossil evidence, paleontologists have identified over 900 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species (birds) and fossil remains. Through the first half of the 20th century, before birds were recognized as dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Some were herbivorous, others carnivorous. Evidence suggests that all dinosaurs were egg-laying, and that nest-building was a trait shared by many dinosaurs, both avian and non-avian.

While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, and some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor and spines. While the dinosaurs’ modern-day surviving avian lineage (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs (non-avian and avian) were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters (130 feet) and heights of 18 m (59 ft) and were the largest land animals of all time. The misconception that non-avian dinosaurs were uniformly gigantic is based in part on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many dinosaurs were quite small, some measuring about 50 centimeters (20 inches) in length.

The first dinosaur fossils were recognized in the early 19th century, with the name «dinosaur» (meaning «terrible lizard») being coined by Sir Richard Owen in 1842 to refer to these «great fossil lizards».^[7][8][9] Since then, mounted fossil dinosaur skeletons have been major attractions at museums worldwide, and dinosaurs have become an enduring part of popular culture. The large sizes of some dinosaurs, as well as their seemingly monstrous and fantastic nature, have ensured their regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.

Definition

Under phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of the most recent common ancestor (MRCA) of Triceratops and modern birds (Neornithes), and all its descendants.^[10] It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.^[11] Both definitions result in the same set of animals being defined as dinosaurs: «Dinosauria = Ornithischia + Saurischia». This definition includes major groups such as ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (bipedal or quadrupedal herbivores with neck frills), pachycephalosaurians (bipedal herbivores with thick skulls), ornithopods (bipedal or quadrupedal herbivores including «duck-bills»), theropods (mostly bipedal carnivores and birds), and sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails).^[12]

Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy; this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well. Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct.^[13] Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.^[14]

Research by Matthew G. Baron, David B. Norman, and Paul M. Barrett in 2017 suggested a radical revision of dinosaurian systematics. Phylogenetic analysis by Baron et al. recovered the Ornithischia as being closer to the Theropoda than the Sauropodomorpha, as opposed to the traditional union of theropods with sauropodomorphs. They resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda. Dinosauria itself was re-defined as the last common ancestor of Triceratops horridus, Passer domesticus and Diplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.^[15][16]

General description

Using one of the above definitions, dinosaurs can be generally described as archosaurs with hind limbs held erect beneath the body.^[17] Other prehistoric animals, including pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs.^[18] Pterosaurs are distantly related to dinosaurs, being members of the clade Ornithodira. The other groups mentioned are, like dinosaurs and pterosaurs, members of Sauropsida (the reptile and bird clade), except Dimetrodon (which is a synapsid). None of them had the erect hind limb posture characteristic of true dinosaurs.^[19]

Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic Era, especially the Jurassic and Cretaceous periods. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a domestic cat, and were generally rodent-sized carnivores of small prey.^[20] They have always been recognized as an extremely varied group of animals; over 900 non-avian dinosaur genera have been identified with certainty as of 2018, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered, and 1124 species by 2016.^[21][22][23] A 1995 study predicted that about 3,400 dinosaur genera ever existed, including many that would not have been preserved in the fossil record.^[24]

In 2016, the estimated number of dinosaur species that existed in the Mesozoic was 1,543–2,468.^[25][26] In 2021, the number of modern-day birds (avian dinosaurs) was estimated to be at 10,806 species.^[27] Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. While dinosaurs were ancestrally bipedal (as are all modern birds), some prehistoric species were quadrupeds, and others, such as Anchisaurus and Iguanodon, could walk just as easily on two or four legs. Cranial modifications like horns and crests are common dinosaurian traits, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size. Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the Early Jurassic epoch.^[28] Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avian dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semiaquatic habits.^[29]

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of their distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton, or are clearly descendants of older dinosaurs showing these modifications. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria; the earliest dinosaurs had them and passed them on to their descendants. Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group.^[30]

A detailed assessment of archosaur interrelations by Sterling Nesbitt^[31] confirmed or found the following twelve unambiguous synapomorphies, some previously known:

• In the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra, the main opening in the rear skull roof
• Epipophyses, obliquely backward-pointing processes on the rear top corners of the anterior (front) neck vertebrae behind the atlas and axis, the first two neck vertebrae
• Apex of a deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)
• Radius, a lower arm bone, shorter than 80% of humerus length
• Fourth trochanter (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thigh bone) is a sharp flange
• Fourth trochanter asymmetrical, with distal, lower, margin forming a steeper angle to the shaft
• On the astragalus and calcaneum, upper ankle bones, the proximal articular facet, the top connecting surface, for the fibula occupies less than 30% of the transverse width of the element
• Exoccipitals (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity, the inner space of the braincase
• In the pelvis, the proximal articular surfaces of the ischium with the ilium and the pubis are separated by a large concave surface (on the upper side of the ischium a part of the open hip joint is located between the contacts with the pubic bone and the ilium)
• Cnemial crest on the tibia (protruding part of the top surface of the shinbone) arcs anterolaterally (curves to the front and the outer side)
• Distinct proximodistally oriented (vertical) ridge present on the posterior face of the distal end of the tibia (the rear surface of the lower end of the shinbone)
• Concave articular surface for the fibula of the calcaneum (the top surface of the calcaneum, where it touches the fibula, has a hollow profile)

Nesbitt found a number of further potential synapomorphies and discounted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cnemial crest on the tibia and of an ascending process on the astragalus, and many others.^[10]

A variety of other skeletal features are shared by dinosaurs. However, because they are either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsids, dinosaurs ancestrally had two pairs of Infratemporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw.^[32] Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);^[10] and a perforate acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia tupiniquim, for example).^[33][34] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic epoch are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.^[35]

Dinosaurs stand with their hind limbs erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.^[36] This posture is due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.^[37] Their erect posture enabled early dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of «sprawling» reptiles.^[38] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.^[39] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a «pillar-erect» configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.^[39]

History of study

Pre-scientific history

Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese considered them to be dragon bones and documented them as such. For example, Huayang Guo Zhi (華陽國志), a gazetteer compiled by Chang Qu (常璩) during the Western Jin Dynasty (265–316), reported the discovery of dragon bones at Wucheng in Sichuan Province.^[40] Villagers in central China have long unearthed fossilized «dragon bones» for use in traditional medicines.^[41] In Europe, dinosaur fossils were generally believed to be the remains of giants and other biblical creatures.^[42]

Early dinosaur research

Scholarly descriptions of what would now be recognized as dinosaur bones first appeared in the late 17th century in England. Part of a bone, now known to have been the femur of a Megalosaurus,^[43] was recovered from a limestone quarry at Cornwell near Chipping Norton, Oxfordshire, in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his The Natural History of Oxford-shire (1677).^[44] He correctly identified the bone as the lower extremity of the femur of a large animal, and recognized that it was too large to belong to any known species. He, therefore, concluded it to be the femur of a huge human, perhaps a Titan or another type of giant featured in legends.^[45][46] Edward Lhuyd, a friend of Sir Isaac Newton, published Lithophylacii Britannici ichnographia (1699), the first scientific treatment of what would now be recognized as a dinosaur when he described and named a sauropod tooth, «Rutellum impicatum»,^[47][48] that had been found in Caswell, near Witney, Oxfordshire.^[49]

Between 1815 and 1824, the Rev William Buckland, the first Reader of Geology at the University of Oxford, collected more fossilized bones of Megalosaurus and became the first person to describe a non-avian dinosaur in a scientific journal.^[43][50] The second non-avian dinosaur genus to be identified, Iguanodon, was discovered in 1822 by Mary Ann Mantell – the wife of English geologist Gideon Mantell. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas. He published his findings in 1825.^[51][52]

The study of these «great fossil lizards» soon became of great interest to European and American scientists, and in 1842 the English paleontologist Sir Richard Owen coined the term «dinosaur», using it to refer to the «distinct tribe or sub-order of Saurian Reptiles» that were then being recognized in England and around the world.^{[7][8][9][53][54]} The term is derived from Ancient Greek δεινός (deinos) ‘terrible, potent or fearfully great’, and σαῦρος (sauros) ‘lizard or reptile’.^[53][55] Though the taxonomic name has often been interpreted as a reference to dinosaurs’ teeth, claws, and other fearsome characteristics, Owen intended it to also evoke their size and majesty.^[56] Owen recognized that the remains that had been found so far, Iguanodon, Megalosaurus and Hylaeosaurus, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. As clarified by British geologist and historian Hugh Torrens, Owen had given a presentation about fossil reptiles to the British Association for the Advancement of Science in 1841, but reports of the time show that Owen did not mention the word «dinosaur», nor recognize dinosaurs as a distinct group of reptiles in his address. He only introduced the Dinosauria in the revised text version of his talk published in April of 1842.^[7][8] With the backing of Prince Albert, the husband of Queen Victoria, Owen established the Natural History Museum, London, to display the national collection of dinosaur fossils and other biological and geological exhibits.^[57]

Discoveries in North America

In 1858, William Parker Foulke discovered the first known American dinosaur, in marl pits in the small town of Haddonfield, New Jersey. (Although fossils had been found before, their nature had not been correctly discerned.) The creature was named Hadrosaurus foulkii. It was an extremely important find: Hadrosaurus was one of the first nearly complete dinosaur skeletons found (the first was in 1834, in Maidstone, England), and it was clearly a bipedal creature. This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke’s discoveries sparked a wave of interests in dinosaurs in the United States, known as dinosaur mania.^[58]

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. This fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Many valuable dinosaur specimens were damaged or destroyed due to the pair’s rough methods: for example, their diggers often used dynamite to unearth bones. Modern paleontologists would find such methods crude and unacceptable, since blasting easily destroys fossil and stratigraphic evidence. Despite their unrefined methods, the contributions of Cope and Marsh to paleontology were vast: Marsh unearthed 86 new species of dinosaur and Cope discovered 56, a total of 142 new species. Cope’s collection is now at the American Museum of Natural History in New York City, while Marsh’s is at the Peabody Museum of Natural History at Yale University.^[59]

«Dinosaur renaissance» and beyond

World War II caused a pause in palaeontological research; after the war, research attention was also diverted increasingly to fossil mammals rather than dinosaurs, which were seen as sluggish and cold-blooded.^[60][61] At the end of the 1960s, however, the field of dinosaur research experienced a surge in activity that remains ongoing.^[62] Several seminal studies led to this activity. First, John Ostrom discovered the bird-like dromaeosaurid theropod Deinonychus and described it in 1969. Its anatomy indicated that it was an active predator that was likely warm-blooded, in marked contrast to the then-prevailing image of dinosaurs.^[60] Concurrently, Robert T. Bakker published a series of studies that likewise argued for active lifestyles in dinosaurs based on anatomical and ecological evidence (see § Physiology),^[63][64] which were subsequently summarized in his 1986 book The Dinosaur Heresies.^[65]

New revelations were supported by an increase in dinosaur discoveries. Major new dinosaur discoveries have been made by paleontologists working in previously unexplored regions, including India, South America, Madagascar, Antarctica, and most significantly China. Across theropods, sauropodomorphs, and ornithischians, the number of named genera began to increase exponentially in the 1990s.^[21] As of 2008, over 30 new species of dinosaurs were named each year.^[66] At least sauropodomorphs experienced a further increase in the number of named species in the 2010s, with an average of 9.3 new species having been named each year between 2009 and 2020. As a consequence, more sauropodomorphs were named between 1990 and 2020 than in all previous years combined.^[67] These new localities also led to improvements in overall specimen quality, with new species being increasingly named not on scrappy fossils but on more complete skeletons, sometimes from multiple individuals. Better specimens also led to new species being invalidated less frequently.^[66] Asian localities have produced the most complete theropod specimens,^[68] while North American localities have produced the most complete sauropodomorph specimens.^[67]

Prior to the dinosaur renaissance, dinosaurs were mostly classified using the traditional rank-based system of Linnaean taxonomy. The renaissance was also accompanied by the increasingly widespread application of cladistics, a more objective method of classification based on ancestry and shared traits, which has proved tremendously useful in the study of dinosaur systematics and evolution. Cladistic analysis, among other techniques, helps to compensate for an often incomplete and fragmentary fossil record.^[69][70] Reference books summarizing the state of dinosaur research, such as David B. Weishampel and colleagues’ The Dinosauria, made knowledge more accessible^[71] and spurred further interest in dinosaur research. The release of the first and second editions of The Dinosauria in 1990 and 2004, and of a review paper by Paul Sereno in 1998, were accompanied by increases in the number of published phylogenetic trees for dinosaurs.^[72]

Soft tissue and molecular preservation

Dinosaur fossils are not limited to bones, but also include imprints or mineralized remains of skin coverings, organs, and other tissues. Of these, skin coverings based on keratin proteins are most easily preserved because of their cross-linked, hydrophobic molecular structure.^[73] Fossils of keratin-based skin coverings or bony skin coverings are known from most major groups of dinosaurs. Dinosaur fossils with scaly skin impressions have been found since the 19th century. Samuel Beckles discovered a sauropod forelimb with preserved skin in 1852 that was incorrectly attributed to a crocodile; it was correctly attributed by Marsh in 1888 and subject to further study by Reginald Hooley in 1917.^[74] Among ornithischians, in 1884 Jacob Wortman found skin impressions on the first known specimen of Edmontosaurus annectens, which were largely destroyed during the specimen’s excavation.^[75] Owen and Hooley subsequently described skin impressions of Hypsilophodon and Iguanodon in 1885 and 1917.^[74] Since then, scale impressions have been most frequently found among hadrosaurids, where the impressions are known from nearly the entire body across multiple specimens.^[76]

Starting from the 1990s, major discoveries of exceptionally preserved fossils in deposits known as conservation Lagerstätten contributed to research on dinosaur soft tissues.^[77][78] Chiefly among these were the rocks that produced the Jehol (Early Cretaceous) and Yanliao (Mid-to-Late Jurassic) biotas of northeastern China, from which hundreds of dinosaur specimens bearing impressions of feather-like structures (both closely related to birds and otherwise, see § Origin of birds) have been described by Xing Xu and colleagues.^[79][80] In living reptiles and mammals, pigment-storing cellular structures known as melanosomes are partially responsible for producing colouration.^[81][82] Both chemical traces of melanin and characteristically-shaped melanosomes have been reported from feathers and scales of Jehol and Yanliao dinosaurs, including both theropods and ornithischians.^[83] This has enabled multiple full-body reconstructions of dinosaur colouration, such as for Sinosauropteryx^[84] and Psittacosaurus^[85] by Jakob Vinther and colleagues, and similar techniques have also been extended to dinosaur fossils from other localities.^[81] (However, some researchers have also suggested that fossilized melanosomes represent bacterial remains.^[86][87]) Stomach contents in some Jehol and Yanliao dinosaurs closely related to birds have also provided indirect indications of diet and digestive system anatomy (e.g., crops).^[88][89] More concrete evidence of internal anatomy has been reported in Scipionyx from the Pietraroja Plattenkalk of Italy. It preserves portions of the intestines, colon, liver, muscles, and windpipe.^[90]

Concurrently, a line of work led by Mary Higby Schweitzer, Jack Horner, and colleagues reported various occurrences of preserved soft tissues and proteins within dinosaur bone fossils. Various mineralized structures that likely represented red blood cells and collagen fibres had been found by Schweitzer and others in tyrannosaurid bones as early as 1991.^[91][92][93] However, in 2005, Schweitzer and colleagues reported that a femur of Tyrannosaurus preserved soft, flexible tissue within, including blood vessels, bone matrix, and connective tissue (bone fibers) that had retained their microscopic structure.^[94] This discovery suggested that original soft tissues could be preserved over geological time.^[73] Later, in 2009, Schweitzer and colleagues reported that a Brachylophosaurus femur preserved similar microstructures, and immunohistochemical techniques (based on antibody binding) demonstrated the presence of proteins such as collagen, elastin, and laminin.^[95] Both specimens yielded collagen protein sequences that were viable for molecular phylogenetic analyses, which grouped them with birds as would be expected.^[95][96] The extraction of fragmentary DNA has also been reported for both of these fossils,^[97] along with a specimen of Hypacrosaurus.^[98] In 2015, Sergio Bertazzo and colleagues reported the preservation of collagen fibres and red blood cells in eight Cretaceous dinosaur specimens that did not show any signs of exceptional preservation, indicating that soft tissue may be preserved more commonly than previously thought.^[99] Suggestions that these structures represent bacterial biofilms^[100] have been rejected,^[101] but cross-contamination remains a possibility that is difficult to detect.^[102]

Evolutionary history

Origins and early evolution

Dinosaurs diverged from their archosaur ancestors during the Middle to Late Triassic epochs, roughly 20 million years after the devastating Permian–Triassic extinction event wiped out an estimated 96% of all marine species and 70% of terrestrial vertebrate species approximately 252 million years ago.^[103][104] The oldest dinosaur fossils known from substantial remains date to the Carnian epoch of the Triassic period, which have been primarily found in the Ischigualasto and Santa Maria Formations of Argentina, and the Pebbly Arkose Formation of Zimbabwe.^[105]

The Ischigualasto Formation (radiometrically dated to be approximately 231-230 million years old^[106]) has produced the early saurischian Eoraptor, originally considered a member of the Herrerasauridae^[107] but now considered to be an early sauropodomorph, along with the herrerasaurids Herrerasaurus and Sanjuansaurus, and the sauropodomorphs Chromogisaurus, Eodromaeus, and Panphagia.^[108] Eoraptor‘s likely resemblance to the common ancestor of all dinosaurs suggests that the first dinosaurs would have been small, bipedal predators.^{[109][110][111]} The Santa Maria Formation (radiometrically dated to be older, at 233.23 million years old^[112]) has produced the herrerasaurids Gnathovorax and Staurikosaurus, along with the sauropodomorphs Bagualosaurus, Buriolestes, Guaibasaurus, Macrocollum, Nhandumirim, Pampadromaeus, Saturnalia, and Unaysaurus.^[108] The Pebbly Arkose Formation, which is of uncertain age but was likely comparable to the other two, has produced the sauropodomorph Mbiresaurus, along with an unnamed herrerasaurid.^[105]

Less well-preserved remains of the sauropodomorphs Jaklapallisaurus and Nambalia, along with the early saurischian Alwalkeria, are respectively known from the Upper Maleri and Lower Maleri Formations of India.^[113] Meanwhile, the Carnian-aged Chañares Formation of Argentina preserves primitive, dinosaur-like ornithodirans such as Lagosuchus and Lagerpeton in Argentina, making it another important site for understanding dinosaur evolution. These ornithodirans support the model of early dinosaurs as small, bipedal predators.^[108][114] Dinosaurs may have appeared as early as the Anisian epoch of the Triassic, approximately 245 million years ago, which is the age of Nyasasaurus from the Manda Formation of Tanzania. However, its known fossils are too fragmentary to tell if it was a dinosaur or only a close relative.^[115] The referral of the Manda Formation to the Anisian is also uncertain. Regardless, dinosaurs existed alongside non-dinosaurian ornithodirans for a period of time, with estimates ranging from 5–10 million years^[116] to 21 million years.^[112]

When dinosaurs appeared, they were not the dominant terrestrial animals. The terrestrial habitats were occupied by various types of archosauromorphs and therapsids, like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchians, such as aetosaurs, ornithosuchids and rauisuchians, which were more successful than the dinosaurs.^[117] Most of these other animals became extinct in the Triassic, in one of two events. First, at about 215 million years ago, a variety of basal archosauromorphs, including the protorosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 201 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. Rhynchosaurs and dicynodonts survived (at least in some areas) at least as late as early –mid Norian and late Norian or earliest Rhaetian stages, respectively,^[118][119] and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.^[10] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, possibly by occupying the niches of the groups that became extinct.^[12] Also notably, there was a heightened rate of extinction during the Carnian pluvial event.^[120]

Evolution and paleobiogeography

Dinosaur evolution after the Triassic followed changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and early sauropodomorph herbivores.^[121] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Early sauropodomorphs did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract.^[122] The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, megalosauroids, and allosauroids, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized metriacanthosaurid theropods and unusual, long-necked sauropods like Mamenchisaurus.^[121] Ankylosaurians and ornithopods were also becoming more common, but primitive sauropodomorphs had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like earlier sauropodomorphs, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.^[122] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.^[14]

By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians.^[121] The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with dental batteries, taken to its extreme in hadrosaurids.^[122] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.^[123]

There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting supercontinent Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.^[121] Flowering plants were greatly radiating,^[122] with the first grasses appearing by the end of the Cretaceous.^[124] Grinding hadrosaurids and shearing ceratopsians became very diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common.^[122]

The Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago at the end of the Cretaceous, caused the extinction of all dinosaur groups except for the neornithine birds. Some other diapsid groups, including crocodilians, dyrosaurs, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event.^[125]

The surviving lineages of neornithine birds, including the ancestors of modern ratites, ducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogene period, entering ecological niches left vacant by the extinction of Mesozoic dinosaur groups such as the arboreal enantiornithines, aquatic hesperornithines, and even the larger terrestrial theropods (in the form of Gastornis, eogruiids, bathornithids, ratites, geranoidids, mihirungs, and «terror birds»). It is often stated that mammals out-competed the neornithines for dominance of most terrestrial niches but many of these groups co-existed with rich mammalian faunas for most of the Cenozoic Era.^[126] Terror birds and bathornithids occupied carnivorous guilds alongside predatory mammals,^[127][128] and ratites are still fairly successful as mid-sized herbivores; eogruiids similarly lasted from the Eocene to Pliocene, only becoming extinct very recently after over 20 million years of co-existence with many mammal groups.^[129]

Classification

Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side.^[30]

Collectively, dinosaurs as a clade are divided into two primary branches, Saurischia and Ornithischia. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Early saurischians—»lizard-hipped», from the Greek sauros (σαῦρος) meaning «lizard» and ischion (ἰσχίον) meaning «hip joint»—retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.^[37] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,^[130] therizinosauroids,^[131] dromaeosaurids,^[132] and birds^[14]). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups).^[29][133]

By contrast, ornithischians—»bird-hipped», from the Greek ornitheios (ὀρνίθειος) meaning «of a bird» and ischion (ἰσχίον) meaning «hip joint»—had a pelvis that superficially resembled a bird’s pelvis: the pubic bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of species that were primarily herbivores.

Despite the terms «bird hip» (Ornithischia) and «lizard hip» (Saurischia), birds are not part of Ornithischia. Birds instead belong to Saurischia, the “lizard-hipped” dinosaurs—birds evolved from earlier dinosaurs with «lizard hips».^[30]

Taxonomy

The following is a simplified classification of dinosaur groups based on their evolutionary relationships, and those of the main dinosaur groups Theropoda, Sauropodomorpha and Ornithischia, compiled by Justin Tweet.^[134] Further details and other hypotheses of classification may be found on individual articles.

• Dinosauria

• †Ornithischia («bird-hipped»; diverse bipedal and quadrupedal herbivores)

• †Heterodontosauridae (small herbivores/omnivores with prominent canine-like teeth)

• †Genasauria («cheeked lizards»)

• †Thyreophora (armored dinosaurs; bipeds and quadrupeds)

• †Eurypoda (heavy, quadrupedal thyreophorans)

• †Stegosauria (spikes and plates as primary armor)

• †Huayangosauridae (small stegosaurs with flank osteoderms and tail clubs)
• †Stegosauridae (large stegosaurs)

• †Ankylosauria (scutes as primary armor)

• †Parankylosauria (small, southern ankylosaurs with macuahuitl-like tails)
• †Nodosauridae (mostly spiky, club-less ankylosaurs)
• †Ankylosauridae (characterized by flat scutes)

• †Ankylosaurinae (club-tailed ankylosaurids)

• †Neornithischia («new ornithischians»)

• †Cerapoda («horned feet»)

• †Marginocephalia (characterized by a cranial growth)

• †Pachycephalosauria (bipeds with domed or knobby growth on skulls)
• †Ceratopsia (bipeds and quadrupeds; many had neck frills and horns)

• †Chaoyangsauridae (small, frill-less basal ceratopsians)
• †Neoceratopsia («new ceratopsians»)

• †Leptoceratopsidae (little to no frills, hornless, with robust jaws)
• †Protoceratopsidae (basal ceratopsians with small frills and stubby horns)

• †Ceratopsoidea (large-horned ceratopsians)

• †Ceratopsidae (large, elaborately ornamented ceratopsians)

• †Chasmosaurinae (ceratopsids with enlarged brow horns)

• †Triceratopsini (very large chasmosaurines with long brow horns)

• †Centrosaurinae (ceratopsids mostly characterized by frill and nasal ornamentation)

• †Nasutoceratopsini (centrosaurines with enlarged nasal cavities)
• †Centrosaurini (centrosaurines with enlarged nasal horns)
• †Pachyrhinosaurini (mostly had nasal bosses instead of horns)

• †Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)

• †Jeholosauridae (small Asian neornithischians)
• †Thescelosauridae («wondrous lizards»)

• †Orodrominae (burrowers)
• †Thescelosaurinae (large thescelosaurids)

• †Iguanodontia («iguana teeth»; advanced ornithopods)

• †Elasmaria (mostly southern ornithopods with mineralized plates along the ribs; may be thescelosaurids)
• †Rhabdodontomorpha (with distinctive dentition)

• †Rhabdodontidae (European rhabdodontomorphs)

• †Dryosauridae (mid-sized, small headed)
• †Camptosauridae (mid-sized, stocky)

• †Styracosterna («spiked sterna»)

• †Hadrosauriformes (ancestrally had a thumb spike)

• †Hadrosauroidea (large quadrupedal herbivores, with teeth merged into dental batteries)

• †Hadrosauromorpha (hadrosaurids and their closest relatives)

• †Hadrosauridae («duck-billed dinosaurs»; often with crests)

• †Saurolophinae (hadrosaurids with solid, small, no crests)

• †Brachylophosaurini (short-crested)
• †Kritosaurini (enlarged, solid nasal crests)
• †Saurolophini (small, spike-like crests)
• †Edmontosaurini (flat-headed saurolophines)

• †Lambeosaurinae (hadrosaurids often with hollow crests)

• †Aralosaurini (solid-crested)
• †Tsintaosaurini (vertical, tube-like crests)
• †Parasaurolophini (long, backwards-arcing crests)
• †Lambeosaurini (usually rounded crests)

• Saurischia

• †Herrerasauridae (early bipedal carnivores)

• †Sauropodomorpha (herbivores with small heads, long necks, and long tails)

• †Unaysauridae (primitive, strictly bipedal «prosauropods»)
• †Plateosauria (diverse; bipeds and quadrupeds)

• †Massospondylidae (long-necked, primitive sauropodomorphs)
• †Riojasauridae (large, primitive sauropodomorphs)

• †Sauropodiformes (heavy, bipeds and quadrupeds)

• †Sauropoda (very large and heavy; quadrupedal)

• †Lessemsauridae (gigantic yet lacking several weight-saving adaptations)
• †Gravisauria («heavy lizards»)

• †Eusauropoda («true sauropods»)

• †Turiasauria (often large, widespread sauropods)
• †Neosauropoda («new sauropods»; columnar limbs)

• †Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)

• †Rebbachisauridae (short-necked, low-browsing diplodocoids often with high backs)
• †Flagellicaudata (whip-tailed)

• †Dicraeosauridae (small, short-necked diplodocoids with enlarged cervical and dorsal vertebrae)
• †Diplodocidae (extremely long-necked)

• †Apatosaurinae (robust cervical vertebrae)
• †Diplodocinae (long, thin necks)

• †Macronaria (boxy skulls; spoon- or pencil-shaped teeth)

• †Titanosauriformes («titan lizard forms»)

• †Brachiosauridae (long-necked, long-armed macronarians)
• †Somphospondyli («porous vertebrae»)

• †Euhelopodidae (stocky, mostly Asian)
• †Titanosauria (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)

• Theropoda (carnivorous)

• Neotheropoda («new theropods»)

• †Coelophysoidea (early theropods; includes Coelophysis and close relatives)
• †»Dilophosaur-grade neotheropods» (larger kink-snouted dinosaurs)
• Averostra («bird snouts»)

• †Ceratosauria (generally elaborately horned carnivores that existed from the Jurassic to Cretaceous periods, originally included Coelophysoidea)

• †Ceratosauridae (ceratosaurs with large teeth)
• †Abelisauroidea (ceratosaurs exemplified by reduced arms and hands)

• †Abelisauridae (large abelisauroids with short arms and oftentimes elaborate facial ornamentation)
• †Noasauridae (diverse, generally light theropods; may include several obscure taxa)

• †Elaphrosaurinae (bird-like; omnivorous as juveniles but herbivorous as adults)
• †Noasaurinae (small carnivores)

• Tetanurae (stiff-tailed dinosaurs)

• †Megalosauroidea (early group of large carnivores)

• †Piatnitzkysauridae (small basal megalosauroids endemic to the Americas)
• †Megalosauridae (large megalosauroids with powerful arms and hands)
• †Spinosauridae (crocodile-like, semiaquatic carnivores)

• Avetheropoda («bird theropods»)

• †Megaraptora (theropods with large hand claws; either carnosaurs or coelurosaurs, potentially tyrannosauroids)
• †Carnosauria (large meat-eating dinosaurs; megalosauroids sometimes included)

• †Metriacanthosauridae (primitive Asian allosauroids)
• †Allosauridae (Allosaurus and its very closest relatives)
• †Carcharodontosauria (robust allosauroids)

• †Carcharodontosauridae (includes some of the largest purely terrestrial carnivores)
• †Neovenatoridae («new hunters»; may include megaraptorans)

• Coelurosauria (feathered theropods, with a range of body sizes and niches)

• †»Nexus of basal coelurosaurs» (used by Tweet to denote well-known taxa with unstable positions at the base of Coelurosauria)
• Tyrannoraptora («tyrant thieves»)

• †Compsognathidae (small early coelurosaurs with short forelimbs)
• †Tyrannosauroidea (mostly large, primitive coelurosaurs)

• †Proceratosauridae (tyrannosauroids with head crests)
• †Tyrannosauridae (Tyrannosaurus and close relatives)

• Maniraptoriformes (bird-like dinosaurs)

• †Ornithomimosauria (small-headed, mostly toothless, omnivorous or possible herbivores)

• †Ornithomimidae (very ostrich-like dinosaurs)

• Maniraptora (dinosaurs with pennaceous feathers)

• †Alvarezsauroidea (small hunters with reduced forelimbs)

• †Alvarezsauridae (insectivores with only one enlarged digit)

• †Therizinosauria (tall, long-necked theropods; omnivores and herbivores)

• †Therizinosauroidea (larger therizinosaurs)

• †Therizinosauridae (sloth-like herbivores, often with enlarged claws)

• †Oviraptorosauria (omnivorous, beaked dinosaurs)

• †Caudipteridae (bird-like, basal oviraptorosaurs)
• †Caenagnathoidea (cassowary-like oviraptorosaurs)

• †Caenagnathidae (toothless oviraptorosaurs known from North America and Asia)
• †Oviraptoridae (characterized by two bony projections at the back of the mouth; exclusive to Asia)

• Paraves (avialans and their closest relatives)

• †Scansoriopterygidae (small tree-climbing theropods with membranous wings)
• †Deinonychosauria (toe-clawed dinosaurs; may not form a natural group)

• †Archaeopterygidae (small, winged theropods or primitive birds)
• †Troodontidae (omnivores; enlarged brain cavities)
• †Dromaeosauridae («raptors»)

• †Microraptoria (characterized by large wings on both the arms and legs; may have been capable of powered flight)
• †Eudromaeosauria (hunters with greatly enlarged sickle claws)

• †Unenlagiidae (piscivores; may be dromaeosaurids)

• †Halszkaraptorinae (duck-like; potentially semiaquatic)
• †Unenlagiinae (long-snouted)

• Avialae (modern birds and extinct relatives)

Timeline of major groups

Timeline of major dinosaur groups per Holtz (2007).

Paleobiology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and other soft tissues.^[90][94] Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics), chemistry, biology, and the Earth sciences (of which paleontology is a sub-discipline).^[135][136] Two topics of particular interest and study have been dinosaur size and behavior.^[137]

Size

Current evidence suggests that dinosaur average size varied through the Triassic, Early Jurassic, Late Jurassic and Cretaceous.^[110] Predatory theropod dinosaurs, which occupied most terrestrial carnivore niches during the Mesozoic, most often fall into the 100 to 1000 kg (220 to 2200 lb) category when sorted by estimated weight into categories based on order of magnitude, whereas recent predatory carnivoran mammals peak in the 10 to 100 kg (22 to 220 lb) category.^[138] The mode of Mesozoic dinosaur body masses is between 1 to 10 metric tons (1.1 to 11.0 short tons).^[139] This contrasts sharply with the average size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kg (4.4 to 11.0 lb).^[140]

The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest was an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as Paraceratherium (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size.^[141] There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.^[142]

Largest and smallest

Scientists will probably never be certain of the largest and smallest dinosaurs to have ever existed. This is because only a tiny percentage of animals were ever fossilized and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork.^[143]

The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907 and 1912. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde in Berlin;^[144] this mount is 12 meters (39 ft) tall and 21.8 to 22.5 meters (72 to 74 ft) long,^[145][146] and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 lb). The longest complete dinosaur is the 27 meters (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh’s Carnegie Museum of Natural History in 1907.^[147] The longest dinosaur known from good fossil material is Patagotitan: the skeleton mount in the American Museum of Natural History in New York is 37 meters (121 ft) long. The Museo Municipal Carmen Funes in Plaza Huincul, Argentina, has an Argentinosaurus reconstructed skeleton mount that is 39.7 meters (130 ft) long.^[148]

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80000 to 100000 kilograms (90 to 110 short tons) and reached lengths of 30 to 40 meters (98 to 131 ft); some of the longest were the 33.5-meter (110 ft) long Diplodocus hallorum^[142] (formerly Seismosaurus), the 33-to-34-meter (108 to 112 ft) long Supersaurus,^[149] and 37-meter (121 ft) long Patagotitan; and the tallest, the 18-meter (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. The heaviest and longest dinosaur may have been Maraapunisaurus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed 122400 kg (270000 lb).^[142] However, as no further evidence of sauropods of this size has been found, and the discoverer, Cope, had made typographic errors before, it is likely to have been an extreme overestimation.^[150]

The largest carnivorous dinosaur was Spinosaurus, reaching a length of 12.6 to 18 meters (41 to 59 ft), and weighing 7 to 20.9 metric tons (7.7 to 23.0 short tons).^[151][152] Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.^[152] Therizinosaurus and Deinocheirus were among the tallest of the theropods. The largest ornithischian dinosaur was probably the hadrosaurid Shantungosaurus giganteus which measured 16.6 meters (54 ft).^[153] The largest individuals may have weighed as much as 16 metric tons (18 short tons).^[154]

The smallest dinosaur known is the bee hummingbird,^[155] with a length of only 5 centimeters (2.0 in) and mass of around 1.8 g (0.063 oz).^[156] The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds.^[157] For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 g (3.9 oz)^[158] and a total skeletal length of 34 centimeters (1.12 ft).^[157][158] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 centimeters (2.0 ft) long each.^[159][160]

Behavior

Many modern birds are highly social, often found living in flocks. There is general agreement that some behaviors that are common in birds, as well as in crocodiles (closest living relatives of birds), were also common among extinct dinosaur groups. Interpretations of behavior in fossil species are generally based on the pose of skeletons and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches.^[135]

The first potential evidence for herding or flocking as a widespread behavior common to many dinosaur groups in addition to birds was the 1878 discovery of 31 Iguanodon, ornithischians that were then thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned.^[161] Other mass-death sites have been discovered subsequently. Those, along with multiple trackways, suggest that gregarious behavior was common in many early dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-billed (hadrosaurids) may have moved in great herds, like the American bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxfordshire, England,^[162] although there is no evidence for specific herd structures.^[163] Congregating into herds may have evolved for defense, for migratory purposes, or to provide protection for young. There is evidence that many types of slow-growing dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud.^[164] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.^[165][166] However, this lifestyle is uncommon among modern birds, crocodiles, and other reptiles, and the taphonomic evidence suggesting mammal-like pack hunting in such theropods as Deinonychus and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators.^[167]

The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations.^[168]

From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops,^[169] providing evidence that dinosaurs did indeed attack each other.^[170] Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur; the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived.^[170] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.^[171]

Comparisons between the scleral rings of dinosaurs and modern birds and reptiles have been used to infer daily activity patterns of dinosaurs. Although it has been suggested that most dinosaurs were active during the day, these comparisons have shown that small predatory dinosaurs such as dromaeosaurids, Juravenator, and Megapnosaurus were likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsians, sauropodomorphs, hadrosaurids, ornithomimosaurs may have been cathemeral, active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal.^[172]

Based on fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle.^[173] Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines.^[174] While some early bird-like species may have already been arboreal as well (including dromaeosaurids) such as Microraptor^[175]) most non-avialan dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics, pioneered by Robert McNeill Alexander, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs’ skeletal structure have investigated how fast dinosaurs could run,^[135] whether diplodocids could create sonic booms via whip-like tail snapping,^[176] and whether sauropods could float.^[177]

Communication

Modern birds are known to communicate using visual and auditory signals, and the wide diversity of visual display structures among fossil dinosaur groups, such as horns, frills, crests, sails, and feathers, suggests that visual communication has always been important in dinosaur biology.^[178] Reconstruction of the plumage color of Anchiornis, suggest the importance of color in visual communication in non-avian dinosaurs.^[179] Vocalization in non-avian dinosaurs is less certain. In birds, the larynx plays no role in sound production. Instead they vocalize with novel organ called the syrinx, located further down the trachea.^[180] The earliest remains of a syrinx was found in a specimen of the duck-like Vegavis iaai dated 69 –66 million years ago, and this organ is unlikely to have existed in non-avian dinosaurs.^[181]

Paleontologist Phil Senter has suggested that non-avian dinosaurs relied mostly on visual displays and possibly non-vocal acoustic sounds like hissing, jaw grinding or clapping, splashing and wing beating (possible in winged maniraptoran dinosaurs). He states they were unlikely to have been capable of vocalizing since their closest relatives, crocodilians and birds, use different means to vocalize, the larynx and syrinx respectively, indicating their common ancestor was mute.^[178] Other researchers have countered that vocalizations also exist in turtles, the closest relatives of archosaurs, suggesting that the trait is ancestral to their lineage. In addition, vocal communication in dinosaurs is indicated by the development of advanced hearing in nearly all major groups. Hence the syrinx may have supplemented and then replaced the larynx as a vocal organ rather than there being a «silent period» in bird evolution.^[182]

In 2023, a fossilized larynx was described from a specimen of the ankylosaurid Pinacosaurus. The structure was composed of cricoid and arytenoid cartilages, similar to those of non-avian reptiles. However, the mobile cricoid-arytenoid joint and long arytenoid cartilages would have allowed for air-flow control similar to that of birds, and thus could have made bird-like vocalizations. In addition, the cartilages were ossified, implying that laryngeal ossification is a feature of some non-avian dinosaurs.^[183] A 2016 study concludes that some dinosaurs may have produced closed mouth vocalizations like cooing, hooting and booming. These occur in both reptiles and birds and involve inflating the esophagus or tracheal pouches. Such vocalizations evolved independently in extant archosaurs numerous times, following increases in body size.^[184] The crests of the Lambeosaurini and nasal chambers of ankylosaurids have been suggested to have functioned in acoustic resonance.^[185][186]

Reproductive biology

All dinosaurs laid amniotic eggs. Dinosaur eggs were usually laid in a nest. Most species create somewhat elaborate nests which can be cups, domes, plates, beds scrapes, mounds, or burrows.^[187] Some species of modern bird have no nests; the cliff-nesting common guillemot lays its eggs on bare rock, and male emperor penguins keep eggs between their body and feet. Primitive birds and many non-avialan dinosaurs often lay eggs in communal nests, with males primarily incubating the eggs. While modern birds have only one functional oviduct and lay one egg at a time, more primitive birds and dinosaurs had two oviducts, like crocodiles. Some non-avialan dinosaurs, such as Troodon, exhibited iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid.^[188]

When laying eggs, females grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, is used to make eggshells. A discovery of features in a Tyrannosaurus skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen. Further research has found medullary bone in the carnosaur Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that the production of medullary tissue is a general characteristic of all dinosaurs.^[189]

Another widespread trait among modern birds (but see below in regards to fossil groups and extant megapodes) is parental care for young after hatching. Jack Horner’s 1978 discovery of a Maiasaura («good mother lizard») nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods.^[190] A specimen of the oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993,^[191] which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.^[192] An embryo of the basal sauropodomorph Massospondylus was found without teeth, indicating that some parental care was required to feed the young dinosaurs.^[193] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.^[194]

However, there is ample evidence of precociality or superprecociality among many dinosaur species, particularly theropods. For instance, non-ornithuromorph birds have been abundantly demonstrated to have had slow growth rates, megapode-like egg burying behavior and the ability to fly soon after birth.^{[195][196][197][198]} Both Tyrannosaurus and Troodon had juveniles with clear superprecociality and likely occupying different ecological niches than the adults.^[188] Superprecociality has been inferred for sauropods.^[199]

Genital structures are unlikely to fossilize as they lack scales that may allow preservation via pigmentation or residual calcium phosphate salts. In 2021, the best preserved specimen of a dinosaur’s cloacal vent exterior was described for Psittacosaurus, demonstrating lateral swellings similar to crocodylian musk glands used in social displays by both sexes and pigmented regions which could also reflect a signalling function. However, this specimen on its own does not offer enough information to determine whether this dinosaur had sexual signalling functions; it only supports the possibility. Cloacal visual signalling can occur in either males or females in living birds, making it unlikely to be useful to determine sex for extinct dinosaurs.^[200]

Physiology

Because both modern crocodilians and birds have four-chambered hearts (albeit modified in crocodilians), it is likely that this is a trait shared by all archosaurs, including all dinosaurs.^[201] While all modern birds have high metabolisms and are endothermic («warm-blooded»), a vigorous debate has been ongoing since the 1960s regarding how far back in the dinosaur lineage this trait extended. Various researchers have supported dinosaurs as being endothermic, ectothermic («cold-blooded»), or somewhere in between.^[202] An emerging consensus among researchers is that, while different lineages of dinosaurs would have had different metabolisms, most of them had higher metabolic rates than other reptiles but lower than living birds and mammals,^[203] which is termed mesothermy by some.^[204] Evidence from crocodiles and their extinct relatives suggests that such elevated metabolisms could have developed in the earliest archosaurs, which were the common ancestors of dinosaurs and crocodiles.^[205][206]

After non-avian dinosaurs were discovered, paleontologists first posited that they were ectothermic. This was used to imply that the ancient dinosaurs were relatively slow, sluggish organisms, even though many modern reptiles are fast and light-footed despite relying on external sources of heat to regulate their body temperature. The idea of dinosaurs as ectothermic remained a prevalent view until Robert T. Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968. Bakker specifically used anatomical and ecological evidence to argue that sauropods, which had hitherto been depicted as sprawling aquatic animals with their tails dragging on the ground, were endotherms that lived vigorous, terrestrial lives. In 1972, Bakker expanded on his arguments based on energy requirements and predator-prey ratios. This was one of the seminal results that led to the dinosaur renaissance.^{[63][64][60][207]}

One of the greatest contributions to the modern understanding of dinosaur physiology has been paleohistology, the study of microscopic tissue structure in dinosaurs.^[208][209] From the 1960s forward, Armand de Ricqlès suggested that the presence of fibrolamellar bone—bony tissue with an irregular, fibrous texture and filled with blood vessels—was indicative of consistently fast growth and therefore endothermy. Fibrolamellar bone was common in both dinosaurs and pterosaurs,^[210][211] though not universally present.^[212][213] This has led to a significant body of work in reconstructing growth curves and modeling the evolution of growth rates across various dinosaur lineages,^[214] which has suggested overall that dinosaurs grew faster than living reptiles.^[209] Other lines of evidence suggesting endothermy include the presence of feathers and other types of body coverings in many lineages (see § Feathers); more consistent ratios of the isotope oxygen-18 in bony tissue compared to ectotherms, particularly as latitude and thus air temperature varied, which suggests stable internal temperatures^[215][216] (although these ratios can be altered during fossilization^[217]); and the discovery of polar dinosaurs, which lived in Australia, Antarctica, and Alaska when these places would have had cool, temperate climates.^{[218][219][220][221]}

In saurischian dinosaurs, higher metabolisms were supported by the evolution of the avian respiratory system, characterized by an extensive system of air sacs that extended the lungs and invaded many of the bones in the skeleton, making them hollow.^[222] Such respiratory systems, which may have appeared in the earliest saurischians,^[223] would have provided them with more oxygen compared to a mammal of similar size, while also having a larger resting tidal volume and requiring a lower breathing frequency, which would have allowed them to sustain higher activity levels.^[141] The rapid airflow would also have been an effective cooling mechanism, which in conjunction with a lower metabolic rate^[224] would have prevented large sauropods from overheating. These traits may have enabled sauropods to grow quickly to gigantic sizes.^[225][226] Sauropods may also have benefitted from their size—their small surface area to volume ratio meant that they would have been able to thermoregulate more easily, a phenomenon termed gigantothermy.^[141][227]

Like other reptiles, dinosaurs are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestine. This would have helped them to conserve water.^[203] In most living species, uric acid is excreted along with feces as a semisolid waste.^[228][229] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.^[230] This material, as well as the output of the intestines, emerges from the cloaca.^[231][232] In addition, many species regurgitate pellets,^[233] and fossil pellets are known as early as the Jurassic from Anchiornis.^[234]

The size and shape of the brain can be partly reconstructed based on the surrounding bones. In 1896, Marsh calculated ratios between brain weight and body weight of seven species of dinosaurs, showing that the brain of dinosaurs was proportionally smaller than in today’s crocodiles, and that the brain of Stegosaurus was smaller than in any living land vertebrate. This contributed to the widespread public notion of dinosaurs as being sluggish and extraordinarily stupid. Harry Jerison, in 1973, showed that proportionally smaller brains are expected at larger body sizes, and that brain size in dinosaurs was not smaller than expected when compared to living reptiles.^[235] Later research showed that relative brain size progressively increased during the evolution of theropods, with the highest intelligence – comparable to that of modern birds – calculated for the troodontid Troodon.^[236]

Origin of birds

The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley.^[237] After the work of Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of the idea of them being descendants of generalized thecodonts, with the key piece of evidence being the supposed lack of clavicles in dinosaurs.^[238] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;^[14] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.^[239] In the 1970s, Ostrom revived the dinosaur–bird theory,^[240] which gained momentum in the coming decades with the advent of cladistic analysis,^[241] and a great increase in the discovery of small theropods and early birds.^[32] Of particular note have been the fossils of the Jehol Biota, where a variety of theropods and early birds have been found, often with feathers of some type.^[70][14] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.^[242] They are most closely allied with maniraptoran coelurosaurs.^[14] A minority of scientists, most notably Alan Feduccia and Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann’s basal archosaur proposal,^[243] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.^[244]

Feathers

Feathers are one of the most recognizable characteristics of modern birds, and a trait that was also shared by several non-avian dinosaurs. Based on the current distribution of fossil evidence, it appears that feathers were an ancestral dinosaurian trait, though one that may have been selectively lost in some species.^[245] Direct fossil evidence of feathers or feather-like structures has been discovered in a diverse array of species in many non-avian dinosaur groups,^[70] both among saurischians and ornithischians. Simple, branched, feather-like structures are known from heterodontosaurids, primitive neornithischians,^[246] and theropods,^[247] and primitive ceratopsians. Evidence for true, vaned feathers similar to the flight feathers of modern birds has been found only in the theropod subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds.^[14][248] Feather-like structures known as pycnofibres have also been found in pterosaurs.^[249]

However, researchers do not agree regarding whether these structures share a common origin between lineages (i.e., they are homologous),^[250][251] or if they were the result of widespread experimentation with skin coverings among ornithodirans.^[252] If the former is the case, filaments may have been common in the ornithodiran lineage and evolved before the appearance of dinosaurs themselves.^[245] Research into the genetics of American alligators has revealed that crocodylian scutes do possess feather-keratins during embryonic development, but these keratins are not expressed by the animals before hatching.^[253] The description of feathered dinosaurs has not been without controversy in general; perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that some purported feather-like fossils are the result of the decomposition of collagenous fiber that underlaid the dinosaurs’ skin,^{[254][255][256]} and that maniraptoran dinosaurs with vaned feathers were not actually dinosaurs, but convergent with dinosaurs.^[244][255] However, their views have for the most part not been accepted by other researchers, to the point that the scientific nature of Feduccia’s proposals has been questioned.^[257]

Archaeopteryx was the first fossil found that revealed a potential connection between dinosaurs and birds. It is considered a transitional fossil, in that it displays features of both groups. Brought to light just two years after Charles Darwin’s seminal On the Origin of Species (1859), its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for the small theropod Compsognathus.^[258] Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Many of these specimens were unearthed in the lagerstätten of the Jehol Biota.^[251] If feather-like structures were indeed widely present among non-avian dinosaurs, the lack of abundant fossil evidence for them may be due to the fact that delicate features like skin and feathers are seldom preserved by fossilization and thus often absent from the fossil record.^[259]

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for paleontologists. Areas of the skeleton with important similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, furcula (wishbone), and breast bone. Comparison of bird and dinosaur skeletons through cladistic analysis strengthens the case for the link.^[260]

Soft anatomy

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to a 2005 investigation led by Patrick M. O’Connor. The lungs of theropod dinosaurs (carnivores that walked on two legs and had bird-like feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. «What was once formally considered unique to birds was present in some form in the ancestors of birds», O’Connor said.^[261][262] In 2008, scientists described Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system. CT scanning of Aerosteon’s fossil bones revealed evidence for the existence of air sacs within the animal’s body cavity.^[222][263]

Behavioral evidence

Fossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.^[264] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds. Several deinonychosaur and oviraptorosaur specimens have also been found preserved on top of their nests, likely brooding in a bird-like manner.^[265] The ratio between egg volume and body mass of adults among these dinosaurs suggest that the eggs were primarily brooded by the male, and that the young were highly precocial, similar to many modern ground-dwelling birds.^[266]

Some dinosaurs are known to have used gizzard stones like modern birds. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.^[267]

Extinction of major groups

All non-avian dinosaurs and most lineages of birds^[268] became extinct in a mass extinction event, called the Cretaceous–Paleogene (K-Pg) extinction event, at the end of the Cretaceous period. Above the Cretaceous–Paleogene boundary, which has been dated to 66.038 ± 0.025 million years ago,^[269] fossils of non-avian dinosaurs disappear abruptly; the absence of dinosaur fossils was historically used to assign rocks to the ensuing Cenozoic. The nature of the event that caused this mass extinction has been extensively studied since the 1970s, leading to the development of two mechanisms that are thought to have played major roles: an extraterrestrial impact event in the Yucatán Peninsula, along with flood basalt volcanism in India. However, the specific mechanisms of the extinction event and the extent of its effects on dinosaurs are still areas of ongoing research.^[270] Alongside dinosaurs, many other groups of animals became extinct: pterosaurs, marine reptiles such as mosasaurs and plesiosaurs, several groups of mammals, ammonites (nautilus-like mollusks), rudists (reef-building bivalves), and various groups of marine plankton.^[271][272] In all, approximately 47% of genera and 76% of species on Earth became extinct during the K-Pg extinction event.^[273] The relatively large size of most dinosaurs and the low diversity of small-bodied dinosaur species at the end of the Cretaceous may have contributed to their extinction;^[274] the extinction of the bird lineages that did not survive may also have been caused by a dependence on forest habitats or a lack of adaptations to eating seeds for survival.^[275][276]

Pre-extinction diversity

Just before the K-Pg extinction event, the number of non-avian dinosaur species that existed globally has been estimated at between 628 and 1078.^[277] It remains uncertain whether the diversity of dinosaurs was in gradual decline before the K-Pg extinction event, or whether dinosaurs were actually thriving prior to the extinction. Rock formations from the Maastrichtian epoch, which directly preceded the extinction, have been found to have lower diversity than the preceding Campanian epoch, which led to the prevailing view of a long-term decline in diversity.^{[271][272][278]} However, these comparisons did not account either for varying preservation potential between rock units or for different extents of exploration and excavation.^[270] In 1984, Dale Russell carried out an analysis to account for these biases, and found no evidence of a decline;^[279] another analysis by David Fastovsky and colleagues in 2004 even showed that dinosaur diversity continually increased until the extinction,^[280] but this analysis has been rebutted.^[281] Since then, different approaches based on statistics and mathematical models have variously supported either a sudden extinction^{[270][277][282]} or a gradual decline.^[283][284] End-Cretaceous trends in diversity may have varied between dinosaur lineages: it has been suggested that sauropods were not in decline, while ornithischians and theropods were in decline.^[285][286]

Impact event

The bolide impact hypothesis, first brought to wide attention in 1980 by Walter Alvarez, Luis Alvarez, and colleagues, attributes the K-Pg extinction event to a bolide (extraterrestrial projectile) impact.^[287] Alvarez and colleagues proposed that a sudden increase in iridium levels, recorded around the world in rock deposits at the Cretaceous–Paleogene boundary, was direct evidence of the impact.^[288] Shocked quartz, indicative of a strong shockwave emanating from an impact, was also found worldwide.^[289] The actual impact site remained elusive until a crater measuring 180 km (110 mi) wide was discovered in the Yucatán Peninsula of southeastern Mexico, and was publicized in a 1991 paper by Alan Hildebrand and colleagues.^[290] Now, the bulk of the evidence suggests that a bolide 5 to 15 kilometers (3 to 9+1⁄2 miles) wide impacted the Yucatán Peninsula 66 million years ago, forming this crater^[291] and creating a «kill mechanism» that triggered the extinction event.^{[292][293][294]}

Within hours, the Chicxulub impact would have created immediate effects such as earthquakes,^[295] tsunamis,^[296] and a global firestorm that likely killed unsheltered animals and started wildfires.^[297][298] However, it would also have had longer-term consequences for the environment. Within days, sulphate aerosols released from rocks at the impact site would have contributed to acid rain and ocean acidification.^[299][300] Soot aerosols are thought to have spread around the world over the ensuing months and years; they would have cooled the surface of the Earth by reflecting thermal radiation, and greatly slowed photosynthesis by blocking out sunlight, thus creating an impact winter.^{[270][301][302]} (This role was ascribed to sulphate aerosols until experiments demonstrated otherwise.^[300]) The cessation of photosynthesis would have led to the collapse of food webs depending on leafy plants, which included all dinosaurs save for grain-eating birds.^[276]

Deccan Traps

At the time of the K-Pg extinction, the Deccan Traps flood basalts of India were actively erupting. The eruptions can be separated into three phases around the K-Pg boundary, two prior to the boundary and one after. The second phase, which occurred very close to the boundary, would have extruded 70 to 80% of the volume of these eruptions in intermittent pulses that occurred around 100,000 years apart.^[303][304] Greenhouse gases such as carbon dioxide and sulphur dioxide would have been released by this volcanic activity,^[305][306] resulting in climate change through temperature perturbations of roughly 3 °C (5.4 °F) but possibly as high as 7 °C (13 °F).^[307] Like the Chicxulub impact, the eruptions may also have released sulphate aerosols, which would have caused acid rain and global cooling.^[308] However, due to large error margins in the dating of the eruptions, the role of the Deccan Traps in the K-Pg extinction remains unclear.^{[269][270][309]}

Before 2000, arguments that the Deccan Traps eruptions—as opposed to the Chicxulub impact—caused the extinction were usually linked to the view that the extinction was gradual. Prior to the discovery of the Chicxulub crater, the Deccan Traps were used to explain the global iridium layer;^[305][310] even after the crater’s discovery, the impact was still thought to only have had a regional, not global, effect on the extinction event.^[311] In response, Luis Alvarez rejected volcanic activity as an explanation for the iridium layer and the extinction as a whole.^[312] Since then, however, most researchers have adopted a more moderate position, which identifies the Chicxulub impact as the primary progenitor of the extinction while also recognizing that the Deccan Traps may also have played a role. Walter Alvarez himself has acknowledged that the Deccan Traps and other ecological factors may have contributed to the extinctions in addition to the Chicxulub impact.^[313] Some estimates have placed the start of the second phase in the Deccan Traps eruptions within 50,000 years after the Chicxulub impact.^[314] Combined with mathematical modelling of the seismic waves that would have been generated by the impact, this has led to the suggestion that the Chicxulub impact may have triggered these eruptions by increasing the permeability of the mantle plume underlying the Deccan Traps.^[315][316]

Whether the Deccan Traps were a major cause of the extinction, on par with the Chicxulub impact, remains uncertain. Proponents consider the climatic impact of the sulphur dioxide released to have been on par with the Chicxulub impact, and also note the role of flood basalt volcanism in other mass extinctions like the Permian-Triassic extinction event.^[317][318] They consider the Chicxulub impact to have worsened the ongoing climate change caused by the eruptions.^[319] Meanwhile, detractors point out the sudden nature of the extinction and that other pulses in Deccan Traps activity of comparable magnitude did not appear to have caused extinctions. They also contend that the causes of different mass extinctions should be assessed separately.^[320] In 2020, Alfio Chiarenza and colleagues suggested that the Deccan Traps may even have had the opposite effect: they suggested that the long-term warming caused by its carbon dioxide emissions may have dampened the impact winter from the Chicxulub impact.^[294]

Possible Paleocene survivors

Non-avian dinosaur remains have occasionally been found above the K-Pg boundary. In 2000, Spencer Lucas and colleagues reported the discovery of a single hadrosaur right femur in the San Juan Basin of New Mexico, and described it as evidence of Paleocene dinosaurs. The rock unit in which the bone was discovered has been dated to the early Paleocene epoch, approximately 64.8 million years ago.^[321] If the bone was not re-deposited by weathering action, it would provide evidence that some dinosaur populations may have survived at least half a million years into the Cenozoic.^[322] Other evidence includes the presence of dinosaur remains in the Hell Creek Formation up to 1.3 m (4.3 ft) above the Cretaceous–Paleogene boundary, representing 40,000 years of elapsed time. This has been used to support the view that the K-Pg extinction was gradual.^[323] However, these supposed Paleocene dinosaurs are considered by many other researchers to be reworked, that is, washed out of their original locations and then re-buried in younger sediments.^{[324][325][326]} The age estimates have also been considered unreliable.^[327]

Cultural depictions

By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the popular imagination and become an enduring part of human culture. The entry of the word «dinosaur» into the common vernacular reflects the animals’ cultural importance: in English, «dinosaur» is commonly used to describe anything that is impractically large, obsolete, or bound for extinction.^[328]

Public enthusiasm for dinosaurs first developed in Victorian England, where in 1854, three decades after the first scientific descriptions of dinosaur remains, a menagerie of lifelike dinosaur sculptures was unveiled in London’s Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way.^[329] The enduring popularity of dinosaurs, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars of the 1880s and 1890s, during which a pair of feuding paleontologists made enormous scientific contributions.^[330]

The popular preoccupation with dinosaurs has ensured their appearance in literature, film, and other media. Beginning in 1852 with a passing mention in Charles Dickens‘ Bleak House,^[331] dinosaurs have been featured in large numbers of fictional works. Jules Verne’s 1864 novel Journey to the Center of the Earth, Sir Arthur Conan Doyle’s 1912 book The Lost World, the 1914 animated film Gertie the Dinosaur (featuring the first animated dinosaur), the iconic 1933 film King Kong, the 1954 Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film adaptation are just a few notable examples of dinosaur appearances in fiction. Authors of general-interest non-fiction works about dinosaurs, including some prominent paleontologists, have often sought to use the animals as a way to educate readers about science in general. Dinosaurs are ubiquitous in advertising; numerous companies have referenced dinosaurs in printed or televised advertisements, either in order to sell their own products or in order to characterize their rivals as slow-moving, dim-witted, or obsolete.^[332][333]

See also

• Dinosaur diet and feeding
• Evolutionary history of life
• Lists of dinosaur-bearing stratigraphic units
• List of dinosaur genera
• List of bird genera
• List of birds
• List of informally named dinosaurs
• List of films featuring dinosaurs

Further reading

• University of Southampton (September 29, 2021). «Two New Species of Large Predatory Dinosaur With Crocodile-Like Skulls Discovered on Isle of Wight». SciTechDaily.
• Zhou, Zhonghe (October 2004). «The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence» (PDF). Naturwissenschaften. Berlin: Springer Science+Business Media. 91 (10): 455–471. Bibcode:2004NW…..91..455Z. doi:10.1007/s00114-004-0570-4. ISSN 0028-1042. PMID 15365634. S2CID 3329625. Archived from the original (PDF) on July 21, 2011. Retrieved November 6, 2019.
• Paul, Gregory S. (2002). Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore; London: Johns Hopkins University Press. ISBN 978-0-8018-6763-7. LCCN 2001000242. OCLC 1088130487..
• Stewart, Tabori & Chang (1997). The Humongous Book of Dinosaurs. New York: Stewart, Tabori & Chang. ISBN 978-1-55670-596-0. LCCN 97000398. OCLC 1037269801.
• Sternberg, Charles Mortram (1966) [Original edition published by E. Cloutier, printer to the King, 1946]. Canadian Dinosaurs. Geological Series. Vol. 54 (2nd ed.). Ottawa: National Museum of Canada. LCCN gs46000214. OCLC 1032865683.

Notes

Bibliography

References

Last Updated:
6 Apr 2023
— Updated example sentences

The word dinosaur of course has its roots in the Greek for “terrible lizard”, but Dr Williams said Owen would have meant the word “terrible” to be understood as “awesome” or “fearfully great”.

What does the word dinosaur mean in Greek?

deinos

When was the word dinosaur coined?

1841

What does the term dinosaur mean?

The word ‘dinosaur’ means ‘terrible lizard’, and comes from the Greek words dino (meaning ‘terrible’) and saur (meaning ‘lizard’). When scientists first discovered dinosaur fossils, they imagined them to be the remains of huge lizards from the past. Tyrannosaurus Rex. Previous: Back.

What does a dinosaur mean spiritually?

Symbolism: A dinosaur represents dragon, protection, fear, past, Karmas, strength, courage, nobility and bravery. Stop fearing your past at once if you dream of a beautiful and happy dinosaur. You have not done anything wrong in your life!

What does the T-Rex symbolize?

In dreams, if you were to see a Tyrannosaurus rex, it might symbolize your fears and be in the image of this carnivorous dinosaur. Getting the T-Rex tattoo might symbolize aggression or power by the person that wears it. However, they emit a confidence that others admire and might be a reason for a tattoo.

What does dinosaur mean in a dream?

Dreaming of a dinosaur signifies that it is time to put things behind you and move on in a situation that has been holding you back. It can symbolize outdated attitudes. This may symbolize your desires. To dream of many dinosaurs means you should probably give up your past habits and ways of thinking.

What does the T-Rex emoji mean?

What does 🦖 T-Rex emoji mean? Depicting an image of a friendly-looking Tyrannosaurus Rex dinosaur, the T-rex emoji is used to share information or enthusiasm about dinosaurs in general. Related words: dinosaur emoji.

What does the Kiwi emoji mean?

Depicting the green flesh of the kiwi fruit, the kiwi fruit emoji is used to represent healthy treats, fancy drinks, sweet and summery vibes, and New Zealand, whose people are nicknamed Kiwis.

What does the blue dinosaur emoji mean?

The emoji can be used to express enthusiasm for dinosaurs, of course, as well as a critique of something or someone who is seen as old-fashioned or outdated.

What does mean from a guy?

This emoji can also mean peace and relaxation. We all know it can be hard for guys to show their true emotions. If he’s using this emoji often, it can mean he is relaxed with you, and is ready to open up. However, you must make sure you don’t push him away in your reply. A lot of girls make this mistake by accident.

What is a blue dinosaur?

Blue is a female Velociraptor that appears in Jurassic World and Jurassic World: Fallen Kingdom.

What’s a blue dinosaur?

Whilst chatting with fans of the film over the last few weeks we have discovered that one of the favourite dinosaurs from the whole movie is the Velociraptor known as “Blue”. The Papo Velociraptor model closely resembles the “Jurassic World” Velociraptors.

Why does Owen say no blue?

After the I – Rex died, Blue, possibly the last Raptor from the 4, looked at Owen, his previous alpha. Owen shook his head, knowing he was no longer the alpha. That’s why Blue dissapointingly walked away after the T – Rex. This is why Owen shook his head at Blue at the end of Jurassic World.

Which Raptor kills Hoskins?

Delta

What is the strongest dinosaur?

A: The strongest was probably the biggest, ultrasauros, who was six-stories high. Or, among meat-eaters, T. rex. Q: In the book The Biggest Dinosaurs by Michael Berenstain, it says that the seismosaurus, found in Mexico, might be larger than the ultrasaurus.

What dinosaur can kill a Spinosaurus?

T. rex

Is Giganotosaurus real?

Giganotosaurus was one of the largest meat-eating dinosaurs. It roamed modern-day Argentina during the late Cretaceous Period, about 99.6 to 97 million years ago. For a long time, Tyrannosaurus rex — “king of the dinosaurs” — was thought to be the largest carnivorous dinosaur.

Would a dinosaur eat a human?

rex surely would have been able to eat people. There are fossil bite marks, matching the teeth of T. rex, on the bones of Triceratops and duck-billed dinosaurs such as Edmontosaurus, which were both over 50 times heavier than an average person. But that doesn’t mean we would be hunted to extinction.

Can at Rex eat a human?

Well yes a T-Rex would eat a human you are no different then another dinosaur except brains and the fact you are smaller you would be an easier target and would still taste good because you are meat, and if it was hungry it’s not just gonna leave a perfect meal standing there.

Can a human outrun at Rex?

(Though 12 miles per hour approaches the top speed of a typical human, depending on conditioning—it equates to a 20-second 100 meter dash or a 5-minute mile—the T. rex’s slow acceleration and inspiring teeth would give the average runner a reasonable chance of outsprinting or outmaneuvering the lumbering predator.)

Can a velociraptor kill a human?

Irl a human could easily kill a lone velociraptor, in fact a nice kick would do it. However, the raptor could still do serious damage, latching onto you and cleaving into flesh with its killer claw. It’d take a lot of mauling to kill a human so it’d probably get crushed before it could do that but it’s possible.

Are Raptors really smart?

How smart were velociraptors, really??? Velociraptors were Dromaeosaurids, among the dinosaurs with the very highest level, so they were truly smart among dinosaurs. On this ranking, they were probably a bit smarter than rabbits and not quite as smart as cats and dogs.

How high could Velociraptors jump?

10 feet

How big was a velociraptor compared to a human?

Velociraptor Was About the Size of a Big Chicken This meat-eater weighed only approximately 30 pounds soaking wet (about the same as a good-sized human toddler) and was just 2 feet tall and 6 feet long.

Which dinosaur is fastest?

ostrich

What was the biggest raptor?

Utahraptor

What does the word dinosaur mean?

Sir Richard Owen came up with the name dinosaur in 1841 to describe the fossils of extinct reptiles. He coined the word by combining the Greek words “deinos”, which means terrible, and “sauros”, which means lizard.

What was the first dinosaur to be named?

Megalosaurus

When was the first dinosaur born?

233.23 million years ago

Where was the last dinosaur found?

A new species of dinosaur has been discovered on the Isle of Wight. Palaeontologists at the University of Southampton believe four bones found at Shanklin last year belong to a new species of theropod dinosaur. It lived in the Cretaceous period, 115 million years ago, and is estimated to have been up to 4m (13ft) long.

Answer: Recent prosauropods from Madagascar are the oldest, about 230 million years old. Until recently, the oldest dinosaur found was Eoraptor (from Argentina) – it is about 228 million years old.

What came before dinosaurs?

The age immediately prior to the dinosaurs was called the Permian. Although there were amphibious reptiles, early versions of the dinosaurs, the dominant life form was the trilobite, visually somewhere between a wood louse and an armadillo. In their heyday there were 15,000 kinds of trilobite.

What is the tallest dinosaur?

Sauroposeidon proteles

What was first dinosaurs or Adam and Eve?

The first has dinosaurs, alongside Adam and Eve, living in harmony. The ferociously fanged T. rex is likely to be a vegetarian. Then comes the Fall of Man and an ugly world where dinosaurs prey on each other and the first extinctions occur.

How long was a year in the Old Testament?

360 days

What the Bible says about 50 years?

You shall thus consecrate the fiftieth year and proclaim a release through the land to all its inhabitants. It shall be a jubilee for you (Leviticus 25:1–4, 8–10, NASB).

How old is a God?

They could tell us at least when figurines of gods and cave paintings appeared. I guess not earlier than 200,000 years ago. I’d even say there was no God before the end of the Neolithic age, and that means God is roughly 7,000 years old.

How old is Jesus now?

Using these methods, most scholars assume a date of birth between 6 and 4 BC, and that Jesus’ preaching began around AD 27–29 and lasted one to three years. They calculate the death of Jesus as having taken place between AD 30 and 36.

Does God has a beginning?

Psalm 90:2 says, “From forever in the past to forever in the future, you are God.” — Common English Bible. This God is so unlimited in power that time and space cannot bind Him or define Him. He created a universe that has no beginning and no end (Genesis 1:1).

What is Jesus the God of?

He is the central figure of Christianity, the world’s largest religion. Most Christians believe he is the incarnation of God the Son and the awaited messiah (the Christ) prophesied in the Old Testament.

di·no·saur

 (dī′nə-sôr′)

n.

1.

a. Any of various extinct terrestrial reptiles of the orders Saurischia and Ornithischia that existed during the Mesozoic Era, including both carnivores and herbivores and often reaching a gigantic size.

b. Any of various other large extinct reptiles, such as an ichthyosaur.

2. A relic of the past: «living dinosaurs of the world of vegetation» (John Olmsted).

3. One that is hopelessly outmoded or unwieldy: «The old, big-city teaching hospital is a dinosaur» (Peggy Breault).

---

[New Latin Dīnosauria, group name, from Dīnosaurus, former genus name : Greek deinos, monstrous + Greek sauros, lizard.]

---

di′no·saur′i·an (-sôr′ē-ən) n. & adj.

di′no·sau′ric (-sôr′ĭk) adj.

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

dinosaur

(ˈdaɪnəˌsɔː)

n

1. (Palaeontology) any extinct terrestrial reptile of the orders Saurischia and Ornithischia, many of which were of gigantic size and abundant in the Mesozoic era. See also saurischian, ornithischian Compare pterosaur, plesiosaur

2. a person or thing that is considered to be out of date

[C19: from New Latin dinosaurus, from Greek deinos fearful + sauros lizard]

ˌdinoˈsaurian adj

Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014

di•no•saur

art at Dior

(ˈdaɪ nəˌsɔr)

n.

1. any herbivorous or carnivorous reptile of the extinct orders Saurischia and Ornithischia, of the Mesozoic Era: some were the largest known land animals.

2. something that is unwieldy, outmoded, or unable to adapt to change.

[< New Latin Dinosaurus (1841), orig. a genus name]

di`no•sau′ri•an, adj.

Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.

di·no·saur

(dī′nə-sôr′)

Any of various extinct reptiles that lived mainly during the Mesozoic Era. Dinosaurs were meat-eating or plant-eating, dwelled mostly on land, and varied from the size of a small dog to the largest land animals that ever lived. See more at ornithischian, saurischian. See Note at bird.

The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

ThesaurusAntonymsRelated WordsSynonymsLegend:

Translations

English[edit]

Alternative forms[edit]

• deinosaur (archaic)
• Dinosaur, dinosaurus

Etymology[edit]

From Ancient Greek δεινός (deinós, “terrible, awesome, mighty, fearfully great”) + σαῦρος (saûros, “lizard, reptile”). Coined as Dinosaur(s) and Dinosauria by paleontologist Richard Owen in 1841/1842.

Pronunciation[edit]

• (UK) IPA^(key): /ˈdaɪnəsɔː(ɹ)/
• Hyphenation: di‧no‧saur

Noun[edit]

dinosaur (plural dinosaurs)

1. (sciences) Any of the animals belonging to the clade Dinosauria, especially those that existed during the Triassic, Jurassic and Cretaceous periods and are now extinct. [from c. 1840]
2. (colloquial) Any member of the clade Dinosauria other than birds.
3. (proscribed) Any extinct reptile, not necessarily belonging to Dinosauria, that existed between about 230 million and 65 million years ago.
4. (figuratively, colloquial) Something or someone that is very old or old-fashioned, or is not willing to change and adapt.

   • 1975, Frederick P. Brooks Jr., The Mythical Man-Month, published 1995:

     [The OS/360 linkage editor] is the culmination of years of development of static overlay technique. Yet it is also the last and finest of the dinosaurs, for it belongs to a system in which multiprogramming is the normal mode and dynamic core allocation the basic assumption.

   • 1999, Ron Harbin; Aaron Barker; Anthony L. Smith (lyrics and music), “What About Now”, performed by Lonestar:

     The sign in the window said for sale or trade on the last remaining dinosaur Detroit made.

5. (figuratively, colloquial) Anything no longer in common use or practice.

Usage notes[edit]

Many animals commonly described as dinosaurs do not belong to Dinosauria, and are not true dinosaurs. These include pterosaurs, ichthyosaurs and plesiosaurs. Describing these as dinosaurs is frowned upon in scientific writing but persists in the media and in everyday speech.

Conversely, not all members of Dinosauria became extinct in the Cretaceous–Paleogene extinction event. Those that survived were the ancestors of modern birds, which therefore also belong to Dinosauria. However, birds are not usually described as dinosaurs, except in some popular science writing.

Synonyms[edit]

• (dinosaur excluding birds): nonavian dinosaur
• (person who is very old): fossil, old fart

Derived terms[edit]

Descendants[edit]

• → Hindi: डायनासोर (ḍāynāsor), डाइनोसर (ḍāinosar)
• → Polish: dinozaur
• → Urdu: ڈایناسور‎

Translations[edit]

Further reading[edit]

• dinosaur on Wikipedia.Wikipedia

Norwegian Bokmål[edit]

Alternative forms[edit]

• dinosaurus

Etymology[edit]

From Ancient Greek δεινός (deinós) + σαῦρος (saûros).

Noun[edit]

dinosaur m (definite singular dinosauren, indefinite plural dinosaurer, definite plural dinosaurene)

1. a dinosaur (extinct reptile)

References[edit]

• “dinosaur” in The Bokmål Dictionary.

Further reading[edit]

• dinosaur on the Norwegian Bokmål Wikipedia.Wikipedia ^nb

Norwegian Nynorsk[edit]

Alternative forms[edit]

• dinosaurus

Etymology[edit]

From Ancient Greek δεινός (deinós) + σαῦρος (saûros).

Noun[edit]

dinosaur m (definite singular dinosauren, indefinite plural dinosaurar, definite plural dinosaurane)

1. a dinosaur (extinct reptile)

References[edit]

• “dinosaur” in The Nynorsk Dictionary.

Further reading[edit]

• dinosaur on the Norwegian Nynorsk Wikipedia.Wikipedia ⁿⁿ

Scots[edit]

Noun[edit]

dinosaur (plural dinosaurs)

1. a dinosaur (extinct reptile)

Further reading[edit]

• dinosaur on the Scots Wikipedia.Wikipedia ^sco

Volapük[edit]

Pronunciation[edit]

• IPA^(key): [di.no.sa.ˈuɾ]

Noun[edit]

dinosaur (nominative plural dinosaurs)

1. dinosaur

Declension[edit]

Hyponyms[edit]

• dinosauril
• dinosaurül
• hidinosaur
• jidinosaur
• ledinosaur

Derived terms[edit]

Dinosaurs Temporal range: Late Triassic–Late Cretaceous, 231.4–65.5 Ma PreЄ Є O S D C P T J K Pg N Descendant taxon Aves survives to present
Mounted skeletons of various dinosaurs, each from a different group. Clockwise from top-left are skeletons of: Tyrannosaurus, a predatory theropod; Diplodocus, a large sauropod; Parasaurolophus, a duck-billed ornithopod; Deinonychus, a bird-like dromaeosaurid; Protoceratops, an early ceratopsian; and Stegosaurus, a plated thyreophoran.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Branch:	Dinosauriformes
Superorder:	Dinosauria Owen, 1842
Orders and suborders
†Ornithischia †Ornithopoda †Thyreophora †Marginocephalia Saurischia †Sauropodomorpha Theropoda (including Aves)

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period (about 230 million years ago) until the end of the Cretaceous (about 65 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of all non-avian dinosaurs at the close of the Mesozoic era. The fossil record indicates that birds evolved within theropod dinosaurs during the Jurassic period. Some of them survived the Cretaceous–Paleogene extinction event, including the ancestors of all modern birds. Consequently, in modern classification systems, birds are considered a type of dinosaur—the only group which survived to the present day.^[1][2]

Dinosaurs are a varied group of animals. Birds, at over 9,000 living species, are the most diverse group of vertebrates besides perciform fish.^[3] In turn, paleontologists have identified over 500 distinct genera^[4] and more than 1,000 different species of non-avian dinosaurs.^[5] Dinosaurs are represented on every continent by both extant species and fossil remains.^[6] Some are herbivorous, others carnivorous. Many dinosaurs have been bipedal, and many extinct groups were also quadrupedal, and some were able to shift between these body postures. Many species possess elaborate display structures such as horns or crests, and some prehistoric groups even developed skeletal modifications such as bony armor and spines. Avian dinosaurs have been the planet’s dominant flying vertebrate since the extinction of the pterosaurs, and evidence suggests that all ancient dinosaurs built nests and laid eggs much as avian species do today. Dinosaurs varied widely in size and weight; the smallest adult theropods were less than 100 centimeters (40 inches) long, while the largest sauropods could reach lengths of almost 50 meters (165 feet) and were several stories tall.

Although the word dinosaur means «terrible lizard,» the name is somewhat misleading, as dinosaurs were not lizards. Rather, they were a separate group of reptiles with a distinct upright posture not found in lizards. Through the first half of the 20th century, most of the scientific community believed dinosaurs were sluggish, unintelligent, and cold-blooded. Most research conducted since the 1970s, however, has indicated that dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction, and many groups (especially the carnivores) were among the most intelligent organisms of the time.

Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons or replicas have been major attractions at museums around the world, and dinosaurs have become a part of world culture. Their diversity, the large sizes of some groups, and their seemingly monstrous and fantastic nature have captured the interest and imagination of the general public for over a century. They have been featured in best-selling books and films such as Jurassic Park, and new discoveries are regularly covered by the media.

Etymology

The taxon Dinosauria was formally named in 1842 by paleontologist Sir Richard Owen, who used it to refer to the «distinct tribe or sub-order of Saurian Reptiles» that were then being recognized in England and around the world.^[7]:103 The term is derived from the Greek words δεινός (deinos, meaning «terrible,» «potent,» or «fearfully great») and σαῦρος (sauros, meaning «lizard» or «reptile»).^[7]:103[8] Though the taxonomic name has often been interpreted as a reference to dinosaurs’ teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.^[9]

Modern definition

Formal definitions are written to correspond with scientific conceptions of dinosaurs that predate the modern use of phylogenetics. The continuity of meaning is intended to prevent confusion about what the term «dinosaur» means.

Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of «Triceratops, Neornithes [modern birds], their most recent common ancestor, and all descendants».^[10] It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.^[11] Both definitions result in the same set of animals being defined as dinosaurs, that is «Dinosauria = Ornithischia + Saurischia», which encompasses theropods (mostly bipedal carnivores and birds), ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including «duck-bills»), and, perhaps, sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails).

Many paleontologists note that the point at which sauropodomorphs and theropods diverged may omit sauropodomorphs from the definition for both saurischians and dinosaurs. To avoid the instability of Dinosauria, a more conservative definition of Dinosauria is defined with respect to four anchoring nodes: Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants. This «safer» definition can be expressed as «Dinosauria = Ornithischia + Sauropodomorpha + Theropoda».^[12]

There is a wide consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict phylogenetic nomenclatural definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds would thus be dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.^[13]

From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word «dinosaur» does not include birds. Additionally, referring to dinosaurs that are not birds as «non-avian dinosaurs» is cumbersome. For clarity, this article will use «dinosaur» as a synonym for «non-avian dinosaur». The term «non-avian dinosaur» will be used for emphasis as needed.

General description

Using one of the above definitions, dinosaurs (aside from birds) can be generally described as terrestrial archosaurian reptiles with limbs held erect beneath the body, that existed from the Late Triassic (first appearing in the Carnian faunal stage) to the Late Cretaceous (going extinct at the end of the Maastrichtian).^[14] Many prehistoric animals are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and Dimetrodon, but are not classified scientifically as dinosaurs. Marine reptiles like ichthyosaurs, mosasaurs, and plesiosaurs were neither terrestrial nor archosaurs; pterosaurs were archosaurs but not terrestrial; and Dimetrodon was a Permian animal more closely related to mammals.^[15] Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a cat, and were generally rodent-sized carnivores of small prey.^[16] One notable exception is Repenomamus giganticus, a triconodont weighing between 12 kilograms (26 lb) and 14 kilograms (31 lb) that is known to have eaten small dinosaurs like young Psittacosaurus.^[17]

Dinosaurs were an extremely varied group of animals; according to a 2006 study, over 500 dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.^[4] An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record.^[18]As of September 17, 2008, 1047 different species of dinosaurs have been named.^[5] Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Many had bony armor, or cranial modifications like horns and crests. Although known for large size, many dinosaurs were human-sized or smaller. Dinosaur remains have been found on every continent on Earth, including Antarctica.^[6] No non-avian dinosaurs are known to have lived in marine habitats or in aerial habitats, although it is possible some feathered non-avian theropods were flyers. There is also evidence that some spinosaurids had semi-aquatic habits.^[19]

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs’ distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common features across a taxonomic group are called synapomorphies.

A detailed assessment of archosaur interrelations by S. Nesbitt^[20] confirmed or found the following 12 unambiguous synapomorphies, some previously known:

• in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra
• epipophyses present in anterior neck vertebrae (except atlas and axis)
• apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)
• radius shorter than 80% of humerus length
• fourth trochanter (projection where the caudofemoralis muscle attaches) on the femur (thigh bone) is a sharp flange
• fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft
• on the astragalus and calcaneum the proximal articular facet for fibula occupies less than 30% of the transverse width of the element
• exocciptials (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity
• proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface
• cnemial crest on the tibia (shinbone) arcs anterolaterally
• distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia

Nesbitt found a number of further potential synapomorphies, and discounted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cenmial crest on the tibia and of an ascending process on the astragalus,^[10] and many others.

A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsid reptiles, dinosaurs ancestrally had two pairs of temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw.^[21] Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);^[10] and an acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example).^[22] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.^[23]

Dinosaurs stood erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.^[24] Their posture was due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.^[25] Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of «sprawling» reptiles.^[26] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.^[27] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a «pillar erect» configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.^[27]

Evolutionary history

Origins and early evolution

Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian–Triassic extinction event wiped out an estimated 95% of all life on Earth.^[28][29] Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists believe Eoraptor resembles the common ancestor of all dinosaurs;^[30] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.^[31] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.

When dinosaurs appeared, terrestrial habitats were occupied by various types of basal archosaurs and therapsids, such as aetosaurs, cynodonts, dicynodonts, ornithosuchids, rauisuchias, and rhynchosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about the boundary between the Carnian and Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal archosauromorphs, including the prolacertiforms and rhynchosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.^[10] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, most likely by occupying the niches of the groups that became extinct.

Evolution and paleobiogeography

Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and prosauropod herbivores.^[32] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Prosauropods did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract.^[33] The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.^[32] Ankylosaurians and ornithopods were also becoming more common, but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.^[33] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.^[13]

By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians.^[32] The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with tooth batteries, taken to its extreme in hadrosaurids.^[33] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.^[34]

There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.^[32] Flowering plants were greatly radiating,^[33] with the first grasses appearing by the end of the Cretaceous.^[35] Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common.^[33]

The Cretaceous–Paleogene extinction event, which occurred approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaurs except for the birds. Some other diapsid groups, such as crocodilians, lizards, snakes, sphenodontians, and choristoderans, also survived the event.^[36]

Classification

Dinosaurs (including birds) are archosaurs, like modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side.

Collectively, dinosaurs are usually regarded as a superorder or an unranked clade. They are divided into two orders, Saurischia and Ornithischia, depending upon pelvic structure. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Saurischians («lizard-hipped», from the Greek sauros (σαυρος) meaning «lizard» and ischion (ισχιον) meaning «hip joint») retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.^[25] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,^[37] therizinosauroids,^[38] dromaeosaurids,^[39] and birds^[13]). Saurischia includes the theropods (bipedal and mostly carnivores, except for birds) and sauropodomorphs (long-necked quadrupedal herbivores).

By contrast, ornithischians («bird-hipped», from the Greek ornitheios (ορνιθειος) meaning «of a bird» and ischion (ισχιον) meaning «hip joint») had a pelvis that superficially resembled a bird’s pelvis: the pubis bone was oriented caudally (rear-pointing) Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of herbivores. (NB: the terms «lizard hip» and «bird hip» are misnomers – birds evolved from dinosaurs with «lizard hips».)

• 

  Edmontosaurus pelvis (showing ornithischian structure – left side)

Taxonomy

The following is a simplified classification of dinosaur groups based on their evolutionary relationships. A more detailed version can be found at Dinosaur classification. The cross (†) is used to signify groups with no living members.

• Dinosauria

• Saurischia («lizard-hipped»; includes Theropoda and Sauropodomorpha)

• †Herrerasauridae (early bipedal carnivores)
• Theropoda (all bipedal; most were carnivorous)

• †Coelophysoidea (small, early theropods; includes Coelophysis and close relatives)
• †Ceratosauria (Ceratosaurus and close relatives, such as abelisaurids)
• Tetanurae («stiff tails»; includes most theropods)

• †Megalosauroidea (Megalosaurus, Spinosaurus, and close relatives)
• †Carnosauria (Allosaurus and close relatives, like Carcharodontosaurus)
• Coelurosauria (closely related to birds; diverse, with a range of body sizes and niches)

• †Tyrannosauridae (Tyrannosaurus and close relatives; had reduced forelimbs)
• †Ornithomimosauria («ostrich-mimics»; mostly toothless; carnivores to possible herbivores)
• Maniraptora («hand snatchers»; had long, slender arms and fingers)

• †Therizinosauria (bipedal herbivores with large hand claws and small heads)
• †Oviraptorosauria (mostly toothless; their diet and lifestyle are uncertain)
• †Deinonychosauria (small- to medium-sized; bird-like, with a distinctive toe claw)
• Avialae (flying, feathered dinosaurs; includes Aves)

• †Sauropodomorpha (herbivores with small heads, long necks, long tails)

• †Prosauropoda (early relatives of sauropods; small- to medium-sized; some possibly omnivorous; bipeds and quadrupeds)
• †Sauropoda (very large and heavy, usually over 15 meters (49 feet) long; quadrupedal)

• †Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)
• †Macronaria (boxy skulls; spoon- or pencil-shaped teeth)

• †Brachiosauridae (very long necks; forelimbs longer than hindlimbs)
• †Titanosauria (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)

• †Ornithischia («bird-hipped»; diverse bipedal and quadrupedal herbivores)

• †Heterodontosauridae (small basal ornithopod herbivores/omnivores with prominent canine teeth)
• †Thyreophora (armored dinosaurs; mostly quadrupeds)

• †Ankylosauria (scutes as primary armor; some had club-like tails)
• †Stegosauria (spikes and plates as primary armor)

• †Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)

• †Iguanodontia (herbivores which developed complex chewing mechanisms)

• †Hadrosauridae (duck-billed dinosaurs)

• †Marginocephalia (characterized by a cranial growth)

• †Pachycephalosauria (bipeds with domed or knobby growth on skulls)
• †Ceratopsia (quadrupeds with frills; many also had horns)

Paleobiology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues.^[40][41] Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics), chemistry, biology, and the earth sciences (of which paleontology is a sub-discipline). Two topics of particular interest and study have been dinosaur size and behavior.

Size

Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods.^[30] Theropod dinosaurs, when sorted by estimated weight into categories based on order of magnitude, most often fall into the 100 to 1000 kilogram (220 to 2200 lb) category, whereas recent predatory carnivorans peak in the 10 to 100 kilogram (22 to 220 lb) category.^[42] The mode of dinosaur body masses is between one and ten metric tonnes.^[43] This contrasts sharply with the size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kilograms (5 to 10 lb).^[44]

The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as the Paraceratherium and the Columbian mammoth were dwarfed by the giant sauropods, and only a handful of modern aquatic animals approach or surpass them in size – most notably the blue whale, which reaches up to 173000 kg (381000 lb) and over 30 meters (98 ft) in length.^{[citation needed]} There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.^[45]

Largest and smallest

Scientists will probably never be certain of the largest and smallest dinosaurs. This is because only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork.

The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907–12. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde Berlin;^[46] this mount is 12 meters (39 ft) tall and 22.5 meters (74 ft) long, and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 lb). The longest complete dinosaur is the 27-meter (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh’s Carnegie Natural History Museum in 1907.

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were all discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80000 to 100000 kilograms (90 to 110 short tons); some of the longest were the 33.5 meters (110 ft) long Diplodocus hallorum^[45] (formerly Seismosaurus) and the 33 meters (108 ft) long Supersaurus;^[47] and the tallest, the 18 meters (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. The heaviest and longest of them all may have been Amphicoelias fragillimus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed over 120000 kg (260000 lb).^[45] The largest known carnivorous dinosaur was Spinosaurus, reaching a length of 16 to 18 meters (50 to 60 ft), and weighing in at 8150 kg (18000 lb).^[48] Other large meat-eaters included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.^[49]

Not including modern birds, the smallest known dinosaurs known were about the size of a pigeon.^[50] The theropods Anchiornis and Epidexipteryx both had a total skeletal length of under 35 centimeters (1.1 ft).^[50][51] Anchiornis is currently the smallest dinosaur described from an adult specimen, with an estimated weight of 110 grams.^[51] The smallest herbivorous dinosaurs included Microceratus and Wannanosaurus, at about 60 cm (2 ft) long each.^[52][53]

Behavior

Interpretations of dinosaur behavior are generally based on the pose of body fossils and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches. As such, the current understanding of dinosaur behavior relies on speculation, and will likely remain controversial for the foreseeable future. However, there is general agreement that some behaviors which are common in crocodiles and birds, dinosaurs’ closest living relatives, were also common among dinosaurs.

The first potential evidence of herding behavior was the 1878 discovery of 31 Iguanodon dinosaurs which were then thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned.^[54] Other mass-death sites have been subsequently discovered. Those, along with multiple trackways, suggest that gregarious behavior was common in many dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-bills (hadrosaurids) may have moved in great herds, like the American Bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxfordshire, England,^[55] although there is not evidence for specific herd structures.^[56] Dinosaurs may have congregated in herds for defense, for migratory purposes, or to provide protection for their young. There is evidence that many types of dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded the remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud.^[57] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.^[58][59] However, this lifestyle is uncommon among the modern relatives of dinosaurs (crocodiles and other reptiles, and birds – Harris’s Hawk is a well-documented exception), and the taphonomic evidence suggesting pack hunting in such theropods as Deinonychus and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators.^[60]

Jack Horner’s 1978 discovery of a Maiasaura («good mother dinosaur») nesting ground in Montana demonstrated that parental care continued long after birth among the ornithopods.^[61] There is also evidence that other Cretaceous-era dinosaurs, like Patagonian titanosaurian sauropods (1997 discovery), also nested in large groups.^[62] The Mongolian oviraptorid Citipati was discovered in a chicken-like brooding position in 1993, which may mean it was covered with an insulating layer of feathers that kept the eggs warm.^[63] Parental care is also implied by other finds. For example, the fossilized remains of a grouping of Psittacosaurus has been found, consisting of one adult and 34 juveniles; in this case, the large number of juveniles may be due to communal nesting.^[64] Additionally, a dinosaur embryo (pertaining to the prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaur.^[65] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.^[66] Nests and eggs have been found for most major groups of dinosaurs, and it appears likely that dinosaurs communicated with their young, in a manner similar to modern birds and crocodiles.

The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations.^[67]

From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops,^[68] providing evidence that dinosaurs did indeed attack each other.^[69] Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur; the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived.^[69] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.^[70]

Comparisons between the scleral rings of dinosaurs and modern birds and reptiles have been used to infer daily activity patterns of dinosaurs. Although it has been suggested that most dinosaurs were active during the day, these comparisons have shown that small predatory dinosaurs such as dromaeosaurids, Juravenator, and Megapnosaurus were likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsians, sauropodomorphs, hadrosaurids, ornithomimosaurs may have been cathemeral, active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal.^[71]

Based on current fossil evidence from dinosaurs such as Oryctodromeus, some herbivorous species seem to have led a partially fossorial (burrowing) lifestyle,^[72] and some bird-like species may have been arboreal (tree climbing), most notably primitive dromaeosaurids such as Microraptor^[73] and the enigmatic scansoriopterygids.^[74] However, most dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics, in particular, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs’ skeletal structure have investigated how fast dinosaurs could run,^[75] whether diplodocids could create sonic booms via whip-like tail snapping,^[76] and whether sauropods could float.^[77]

Communication and vocalization

The nature of dinosaur communication remains enigmatic, and is an active area of research. In 2008, paleontologist Phil Senter examined the evidence for vocalization in Mesozoic animal life, including dinosaurs.^[78] Senter found that, contrary to popular depictions of roaring dinosaurs in motion pictures, it is likely that most dinosaurs were not capable of creating any vocalizations. To draw this conclusion, Senter studied the distribution of vocal organs in reptiles and birds. He found that vocal cords in the larynx probably evolved multiple times among reptiles, including crocodilians, which are able to produce guttural roars. Birds, on the other hand, lack a larynx. Instead, bird calls are produced by the syrinx, a vocal organ found only in birds, and which is not related to the larynx, meaning it evolved independently from the vocal organs in reptiles. The syrinx depends on the air sac system in birds to function; specifically, it requires the presence of a clavicular air sac near the wishbone or collar bone. This air sac leaves distinctive marks or opening on the bones, including a distinct opening in the upper arm bone (humerus). While many dinosaurs show evidence of extensive air sac systems, almost none possess the clavicular air sac necessary to vocalize (one exception, Aerosteon, probably evolved its clavicular air sac independently of birds for reasons other than vocalization).^[78]

The most primitive animals with evidence of a vocalizing syrinx are the enantironithine birds. Any bird-line archosaurs more primitive than this probably did not make vocal calls. Rather, several lines of evidence suggest that dinosaurs used primarily visual communication, in the form of distinctive-looking (and possibly brightly colored) horns, frills, crests, sails and feathers. This is similar to some modern reptile groups such as lizards, in which many forms are largely silent (though like dinosaurs they possess well-developed senses of hearing) but use complex coloration and display behaviors to communicate.^[78]

Also, though they may not have been able to vocalize, some dinosaurs may have used other methods of producing sound for communication. Modern animals, including reptiles and birds, use a wide variety of non-vocal sound communication, including hissing, jaw grinding or clapping, use of environment (such as splashing), and wing beating (which would have been possible in winged maniraptoran dinosaurs).^[78]

Some studies have suggested that the hollow crests of the lambeosaurines may have functioned as resonance chambers used for a wide range of vocalizations.^[79][80] However, Senter (2008) noted that such chambers are also used in modern non-vocal animals to accentuate or deepen non-vocal sounds like hissing. For example, many snakes, which lack vocal cords, have resonating chambers in the skull.^[78]

Reproductive biology

A discovery of features in a Tyrannosaurus rex skeleton provided evidence of medullary bone in dinosaurs and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, is used to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of the Tyrannosaurus rex specimen’s hind limb suggested that T. rex used similar reproductive strategies, and revealed the specimen to be female.^[81] Further research has found medullary bone in the theropod Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity rather quickly for such large animals.^[82]

Waste

Like other reptiles, dinosaurs are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestine. In most living species, uric acid is excreted along with feces as a semisolid waste.^[83][84][85] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.^[86] They also excrete creatine, rather than creatinine like mammals. This material, as well as the output of the intestines, emerges from the cloaca.^[87][88] In addition, many species regurgitate pellets, and fossil pellets that may have come from dinosaurs are known from as long ago as the Cretaceous period.^[89]

Physiology

A vigorous debate on the subject of temperature regulation in dinosaurs has been ongoing since the 1960s. Originally, scientists broadly disagreed as to whether dinosaurs were capable of regulating their body temperatures at all. More recently, dinosaur endothermy has become the consensus view, and debate has focused on the mechanisms of temperature regulation.

After dinosaurs were discovered, paleontologists first posited that they were ectothermic creatures: «terrible lizards» as their name suggests. This supposed cold-bloodedness was used to imply that dinosaurs were relatively slow, sluggish organisms, even though many modern reptiles are fast and light-footed despite relying on external sources of heat to regulate their body temperature. The idea of dinosaurs as ectothermic and sluggish remained a prevalent view until Robert T. «Bob» Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968.^{[citation needed]}

Modern evidence indicates that dinosaurs thrived in cooler temperate climates, and that at least some dinosaur species must have regulated their body temperature by internal biological means (aided by the animals’ bulk in large species). Evidence of endothermy in dinosaurs includes the discovery of polar dinosaurs in Australia and Antarctica (where they would have experienced a cold, dark six-month winter), the discovery of dinosaurs whose feathers may have provided regulatory insulation, and analysis of blood-vessel structures within dinosaur bone that are typical of endotherms. Scientific debate continues regarding the specific ways in which dinosaurs regulated their temperature.^[90]

Complicating the debate is the fact that warm-bloodedness can emerge based on more than one mechanism. Most discussions of dinosaur endothermy tend to compare them with average-sized birds or mammals, which expend energy to elevate body temperature above that of the environment. Small birds and mammals also possess insulation, such as fat, fur, or feathers, which slows down heat loss. However, large mammals, such as elephants, face a different problem because of their relatively small ratio of surface area to volume (Haldane’s principle). This ratio compares the volume of an animal with the area of its skin: as an animal gets bigger, its surface area increases more slowly than its volume. At a certain point, the amount of heat radiated away through the skin drops below the amount of heat produced inside the body, forcing animals to use additional methods to avoid overheating. In the case of elephants, they have little hair as adults, have large ears which increase their surface area, and have behavioral adaptations as well (such as using the trunk to spray water on themselves and mud-wallowing). These behaviors increase cooling through evaporation.

Large dinosaurs would presumably have had to deal with similar issues; their body size suggest they lost heat relatively slowly to the surrounding air, and so could have been what are called inertial homeotherms, animals that are warmer than their environments through sheer size rather than through special adaptations like those of birds or mammals. However, so far this theory fails to account for the numerous dog- and goat-sized dinosaur species, or the young of larger species.

Modern computerized tomography (CT) scans of a dinosaur’s chest cavity (conducted in 2000) found the apparent remnants of a four-chambered heart, much like those found in today’s mammals and birds.^[91] The idea is controversial within the scientific community, coming under fire for bad anatomical science^[92] or simply wishful thinking.^[93] The question of how this find reflects on metabolic rate and dinosaur internal anatomy may be moot, though, regardless of the object’s identity: both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), and so dinosaurs probably had them as well.^[94]

Soft tissue and DNA

One of the best examples of soft-tissue impressions in a fossil dinosaur was discovered in Petraroia, Italy. The discovery was reported in 1998, and described the specimen of a small, very young coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.^[40]

In the March 2005 issue of Science, the paleontologist Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team.^[41]

When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone-marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Schweitzer’s discovery, are not yet clear; study and interpretation of the material is ongoing.^[41]

The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but, upon further inspection and peer review, neither of these reports could be confirmed.^[95] However, a functional visual peptide of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds.^[96] In addition, several proteins, including hemoglobin,^[97] have putatively been detected in dinosaur fossils.^[98]

Feathers and the origin of birds

The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley.^[99] After the work of Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of the idea of their being descendants of generalized thecodonts, with the key piece of evidence being the supposed lack of clavicles in dinosaurs.^[100] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;^[13] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.^[101] In the 1970s, John Ostrom revived the dinosaur–bird theory,^[102] which gained momentum in the coming decades with the advent of cladistic analysis,^[103] and a great increase in the discovery of small theropods and early birds.^[21] Of particular note have been the fossils of the Yixian Formation, where a variety of theropods and early birds have been found, often with feathers of some type.^[13] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.^[104] They are most closely allied with maniraptoran coelurosaurs.^[13] A minority of scientists, most notably Alan Feduccia and Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann’s basal archosaur proposal,^[105] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.^[106]

Feathers

Archaeopteryx was the first fossil found which revealed a potential connection between dinosaurs and birds. It is considered a transitional fossil, in that it displays features of both groups. Brought to light just two years after Darwin’s seminal The Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus.^[107]

Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in the lagerstätte of the Yixian Formation, Liaoning, northeastern China, which was part of an island continent during the Cretaceous. Though feathers have been found in only a few locations, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be because delicate features like skin and feathers are not often preserved by fossilization and thus are absent from the fossil record. To this point, protofeathers (thin, filament-like structures) are known from dinosaurs at the base of Coelurosauria, such as compsognathids like Sinosauropteryx and tyrannosauroids (Dilong),^[108] but barbed feathers are known only among the coelurosaur subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds.^[13][109] The description of feathered dinosaurs has not been without controversy; perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that protofeathers are the result of the decomposition of collagenous fiber that underlaid the dinosaurs’ integument,^{[110][111][112]} and that maniraptoran dinosaurs with barbed feathers were not actually dinosaurs, but convergent with dinosaurs.^[106][111] However, their views have for the most part not been accepted by other researchers, to the point that the question of the scientific nature of Feduccia’s proposals has been raised.^[113]

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for paleontologists. Areas of the skeleton with important similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, furcula (wishbone), and breast bone. Comparison of bird and dinosaur skeletons through cladistic analysis strengthens the case for the link.

Soft anatomy

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was led by Patrick O’Connor of Ohio University. The lungs of theropod dinosaurs (carnivores that walked on two legs and had bird-like feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. «What was once formally considered unique to birds was present in some form in the ancestors of birds», O’Connor said.^[114] In a 2008 paper published in the online journal PLoS ONE, scientists described Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system. CT-scanning revealed the evidence of air sacs within the body cavity of the Aerosteon skeleton.^[115][116]

Behavioral evidence

Fossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.^[117] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds. Several deinonychosaur and oviraptorosaur specimens have also been found preserved on top of their nests, likely brooding in a bird-like manner.^[118] The ratio between egg volume and body mass of adults among these dinosaurs suggest that the eggs were primarily brooded by the male, and that the young were highly precocial, similar to many modern ground-dwelling birds.^[119]

Some dinosaurs are known to have used gizzard stones like modern birds. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.^[120]

Extinction of major groups

The discovery that birds are a type of dinosaur showed that dinosaurs in general are not, in fact, extinct as is commonly stated.^[121] However, all non-avian dinosaurs as well as many groups of birds did suddenly become extinct approximately 65 million years ago. Many other groups of animals also became extinct at this time, including ammonites (nautilus-like mollusks), mosasaurs, plesiosaurs, pterosaurs, and many groups of mammals.^[6] This mass extinction is known as the Cretaceous–Paleogene extinction event. The nature of the event that caused this mass extinction has been extensively studied since the 1970s; at present, several related theories are supported by paleontologists. Though the consensus is that an impact event was the primary cause of dinosaur extinction, some scientists cite other possible causes, or support the idea that a confluence of several factors was responsible for the sudden disappearance of dinosaurs from the fossil record.

At the peak of the Mesozoic, there were no polar ice caps, and sea levels are estimated to have been from 100 to 250 meters (300 to 800 ft) higher than they are today. The planet’s temperature was also much more uniform, with only 25 °C (45 °F) separating average polar temperatures from those at the equator. On average, atmospheric temperatures were also much higher; the poles, for example, were 50 °C (90 °F) warmer than today.^[122][123]

The atmosphere’s composition during the Mesozoic was vastly different as well. Carbon dioxide levels were up to 12 times higher than today’s levels, and oxygen formed 32 to 35% of the atmosphere,^{[citation needed]} as compared to 21% today. However, by the late Cretaceous, the environment was changing dramatically. Volcanic activity was decreasing, which led to a cooling trend as levels of atmospheric carbon dioxide dropped. Oxygen levels in the atmosphere also started to fluctuate and would ultimately fall considerably. Some scientists hypothesize that climate change, combined with lower oxygen levels, might have led directly to the demise of many species. If the dinosaurs had respiratory systems similar to those commonly found in modern birds, it may have been particularly difficult for them to cope with reduced respiratory efficiency, given the enormous oxygen demands of their very large bodies.^[6]

Impact event

The asteroid collision theory, which was brought to wide attention in 1980 by Walter Alvarez and colleagues, links the extinction event at the end of the Cretaceous period to a bolide impact approximately 65.5 million years ago. Alvarez et al. proposed that a sudden increase in iridium levels, recorded around the world in the period’s rock stratum, was direct evidence of the impact.^[124] The bulk of the evidence now suggests that a bolide 5 to 15 kilometers (3 to 9 mi) wide hit in the vicinity of the Yucatán Peninsula (in southeastern Mexico), creating the approximately 180 km (110 mi) Chicxulub Crater and triggering the mass extinction.^[125][126] Scientists are not certain whether dinosaurs were thriving or declining before the impact event. Some scientists propose that the meteorite caused a long and unnatural drop in Earth’s atmospheric temperature, while others claim that it would have instead created an unusual heat wave. The consensus among scientists who support this theory is that the impact caused extinctions both directly (by heat from the meteorite impact) and also indirectly (via a worldwide cooling brought about when matter ejected from the impact crater reflected thermal radiation from the sun). Although the speed of extinction cannot be deduced from the fossil record alone, various models suggest that the extinction was extremely rapid, being down to hours rather than years.^[127]

In September 2007, U.S. researchers led by William Bottke of the Southwest Research Institute in Boulder, Colorado, and Czech scientists used computer simulations to identify the probable source of the Chicxulub impact. They calculated a 90% probability that a giant asteroid named Baptistina, approximately 160 km (99 mi) in diameter, orbiting in the asteroid belt which lies between Mars and Jupiter, was struck by a smaller unnamed asteroid about 55 km (35 mi) in diameter about 160 million years ago. The impact shattered Baptistina, creating a cluster which still exists today as the Baptistina family. Calculations indicate that some of the fragments were sent hurtling into earth-crossing orbits, one of which was the 10 km (6.2 mi) wide meteorite which struck Mexico’s Yucatan peninsula 65 million years ago, creating the Chicxulub crater.^[128] In 2011, new data from the Wide-field Infrared Survey Explorer revised the date of the collision which created the Baptistina family to about 80 million years ago. This makes an asteroid from this family highly improbable to be the asteroid that created the Chicxulub Crater, as typically the process of resonance and collision of an asteroid takes many tens of millions of years.^[129]

A similar but more controversial explanation proposes that «passages of the [hypothetical] solar companion star Nemesis through the Oort comet cloud would trigger comet showers.»^[130] One or more of these comets then collided with the Earth at approximately the same time, causing the worldwide extinction. As with the impact of a single asteroid, the end result of this comet bombardment would have been a sudden drop in global temperatures, followed by a protracted cool period.^[130]

Deccan Traps

Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 million years ago and lasted for over 2 million years. However, there is evidence that two thirds of the Deccan Traps were created in only 1 million years about 65.5 million years ago, and so these eruptions would have caused a fairly rapid extinction, possibly over a period of thousands of years, but still longer than would be expected from a single impact event.^[131][132]

The Deccan Traps could have caused extinction through several mechanisms, including the release into the air of dust and sulphuric aerosols, which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions, which would have increased the greenhouse effect when the dust and aerosols cleared from the atmosphere.^[132] Before the mass extinction of the dinosaurs, the release of volcanic gases during the formation of the Deccan Traps «contributed to an apparently massive global warming. Some data point to an average rise in temperature of 8 °C (14 °F) in the last half million years before the impact [at Chicxulub].»^[131][132]

In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.^[133]

Failure to adapt to changing conditions

Lloyd et al. (2008) noted that, in the Mid Cretaceous, the flowering, angiosperm plants became a major part of terrestrial ecosystems, which had previously been dominated by gymnosperms such as conifers. Dinosaur coprolite–fossilized dung–indicate that, while some ate angiosperms, most herbivorous dinosaurs ate mainly gymnosperms. Statistical analysis by Lloyd et al. concluded that, contrary to earlier studies, dinosaurs did not diversify very much in the Late Cretaceous. Lloyd et al. suggested that dinosaurs’ failure to diversify as ecosystems were changing doomed them to extinction.^[134]

Possible Paleocene survivors

Non-avian dinosaur remains are occasionally found above the K–T boundary. In 2001, paleontologists Zielinski and Budahn reported the discovery of a single hadrosaur leg-bone fossil in the San Juan Basin, New Mexico, and described it as evidence of Paleocene dinosaurs. The formation in which the bone was discovered has been dated to the early Paleocene epoch, approximately 64.5 million years ago. If the bone was not re-deposited into that stratum by weathering action, it would provide evidence that some dinosaur populations may have survived at least a half million years into the Cenozoic Era.^[135] Other evidence includes the finding of dinosaur remains in the Hell Creek Formation up to 1.3 meters (51 in) above (40000 years later than) the K–T boundary. Similar reports have come from other parts of the world, including China.^[136] Many scientists, however, dismissed the supposed Paleocene dinosaurs as re-worked, that is, washed out of their original locations and then re-buried in much later sediments.^[137][138] However, direct dating of the bones themselves has supported the later date, with U–Pb dating methods resulting in a precise age of 64.8 ± 0.9 million years ago.^[139] If correct, the presence of a handful of dinosaurs in the early Paleocene would not change the underlying facts of the extinction.^[137]

History of discovery

Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese, whose modern word for dinosaur is konglong (恐龍, or «terrible dragon»), considered them to be dragon bones and documented them as such. For example, Hua Yang Guo Zhi, a book written by Zhang Qu during the Western Jin Dynasty, reported the discovery of dragon bones at Wucheng in Sichuan Province.^[140] Villagers in central China have long unearthed fossilized «dragon bones» for use in traditional medicines, a practice that continues today.^[141] In Europe, dinosaur fossils were generally believed to be the remains of giants and other creatures killed by the Great Flood.

Scholarly descriptions of what would now be recognized as dinosaur bones first appeared in the late 17th century in England. Part of a bone, now known to have been the femur of a Megalosaurus,^[142] was recovered from a limestone quarry at Cornwell near Chipping Norton, Oxfordshire, England, in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his Natural History of Oxfordshire in 1677. He correctly identified the bone as the lower extremity of the femur of a large animal, and recognized that it was too large to belong to any known species. He therefore concluded it to be the thigh bone of a giant human similar to those mentioned in the Bible. In 1699, Edward Lhuyd, a friend of Sir Isaac Newton, was responsible for the first published scientific treatment of what would now be recognized as a dinosaur when he described and named a sauropod tooth, «Rutellum implicatum»,^[143][144] that had been found in Caswell, near Witney, Oxfordshire.^[145]

Between 1815 and 1824, the Rev William Buckland, a professor of geology at Oxford University, collected more fossilized bones of Megalosaurus and became the first person to describe a dinosaur in a scientific journal.^[142][146] The second dinosaur genus to be identified, Iguanodon, was discovered in 1822 by Mary Ann Mantell – the wife of English geologist Gideon Mantell. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas. He published his findings in 1825.^[147][148]

The study of these «great fossil lizards» soon became of great interest to European and American scientists, and in 1842 the English paleontologist Richard Owen coined the term «dinosaur». He recognized that the remains that had been found so far, Iguanodon, Megalosaurus and Hylaeosaurus, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. With the backing of Prince Albert of Saxe-Coburg-Gotha, the husband of Queen Victoria, Owen established the Natural History Museum in South Kensington, London, to display the national collection of dinosaur fossils and other biological and geological exhibits.

In 1858, the first known American dinosaur was discovered, in marl pits in the small town of Haddonfield, New Jersey (although fossils had been found before, their nature had not been correctly discerned). The creature was named Hadrosaurus foulkii. It was an extremely important find: Hadrosaurus was one of the first nearly complete dinosaur skeletons found (the first was in 1834, in Maidstone, Kent, England), and it was clearly a bipedal creature. This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke’s discoveries sparked a wave of dinosaur mania in the United States.

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. The feud probably originated when Marsh publicly pointed out that Cope’s reconstruction of an Elasmosaurus skeleton was flawed: Cope had inadvertently placed the plesiosaur’s head at what should have been the animal’s tail end. The fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Marsh ‘won’ the contest primarily because he was better funded through a relationship with the US Geological Survey. Unfortunately, many valuable dinosaur specimens were damaged or destroyed due to the pair’s rough methods: for example, their diggers often used dynamite to unearth bones (a method modern paleontologists would find appalling). Despite their unrefined methods, the contributions of Cope and Marsh to paleontology were vast: Marsh unearthed 86 new species of dinosaur and Cope discovered 56, a total of 142 new species. Cope’s collection is now at the American Museum of Natural History in New York, while Marsh’s is on display at the Peabody Museum of Natural History at Yale University.^[149]

After 1897, the search for dinosaur fossils extended to every continent, including Antarctica. The first Antarctic dinosaur to be discovered, the ankylosaurid Antarctopelta oliveroi, was found on Ross Island in 1986, although it was 1994 before an Antarctic species, the theropod Cryolophosaurus ellioti, was formally named and described in a scientific journal.

Current dinosaur «hot spots» include southern South America (especially Argentina) and China. China in particular has produced many exceptional feathered dinosaur specimens due to the unique geology of its dinosaur beds, as well as an ancient arid climate particularly conducive to fossilization.

The «dinosaur renaissance»

The field of dinosaur research has enjoyed a surge in activity that began in the 1970s and is ongoing. This was triggered, in part, by John Ostrom’s discovery of Deinonychus, an active predator that may have been warm-blooded, in marked contrast to the then-prevailing image of dinosaurs as sluggish and cold-blooded. Vertebrate paleontology has become a global science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including India, South America, Madagascar, Antarctica, and most significantly China (the amazingly well-preserved feathered dinosaurs in China have further consolidated the link between dinosaurs and their conjectured living descendants, modern birds). The widespread application of cladistics, which rigorously analyzes the relationships between biological organisms, has also proved tremendously useful in classifying dinosaurs. Cladistic analysis, among other modern techniques, helps to compensate for an often incomplete and fragmentary fossil record.

Cultural depictions

By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the popular imagination and become an enduring part of human culture. Entry of the word «dinosaur» into the common vernacular reflects the animals’ cultural importance: in English, «dinosaur» is commonly used to describe anything that is impractically large, obsolete, or bound for extinction.^[150]

Public enthusiasm for dinosaurs first developed in Victorian England, where in 1854, three decades after the first scientific descriptions of dinosaur remains, the famous dinosaur sculptures were unveiled in London’s Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way.^[151] Dinosaurs’ enduring popularity, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars of the 1880s and 1890s, during which a pair of feuding paleontologists made enormous scientific contributions.^[152]

The popular preoccupation with dinosaurs has ensured their appearance in literature, film and other media. Beginning in 1852 with a passing mention in Charles Dickens’ Bleak House,^[153] dinosaurs have been featured in large numbers of fictional works. Sir Arthur Conan Doyle’s 1912 book The Lost World, the iconic 1933 film King Kong, 1954’s Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film adaptation are just a few notable examples of dinosaur appearances in fiction. Authors of general-interest non-fiction works about dinosaurs, including some prominent paleontologists, have often sought to use the animals as a way to educate readers about science in general. Dinosaurs are ubiquitous in advertising; numerous companies have referenced dinosaurs in printed or televised advertisements, either in order to sell their own products or in order to characterize their rivals as slow-moving, dim-witted or obsolete.^[154]

See also

• Evolutionary history of life
• List of dinosaurs
• List of dinosaur-bearing rock formations
• Physiology of dinosaurs
• Prehistoric reptile

Notes and references

Further reading

• Bakker, Robert T. (1986). The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction. New York: Morrow. ISBN 0-688-04287-2.
• Holtz, Thomas R. Jr. (2007). Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. New York: Random House. ISBN 978-0-375-82419-7.
• Paul, Gregory S. (2000). The Scientific American Book of Dinosaurs. New York: St. Martin’s Press. ISBN 0-312-26226-4.
• Paul, Gregory S. (2002). Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: The Johns Hopkins University Press. ISBN 0-8018-6763-0.

External links

General

• DinoDatabase.com | Hundreds of dinosaurs and dinosaur related topics.

Images

• The Science and Art of Gregory S. Paul Influential paleontologist’s anatomy art and paintings
• Skeletal Drawing Professional restorations of numerous dinosaurs, and discussions of dinosaur anatomy.

Video

• BBC Nature:Watch dinosaurs brought to life and get experts’ interpretations with videos from BBC programmes including Walking with Dinosaurs

Popular

• Dinosaurs & other extinct creatures: From the Natural History Museum, a well illustrated dinosaur directory.
• Dinosaurnews (www.dinosaurnews.org) The dinosaur-related headlines from around the world. Recent news on dinosaurs, including finds and discoveries, and many links.
• Dinosauria From UC Berkeley Museum of Paleontology Detailed information – scroll down for menu.
• LiveScience.com All about dinosaurs, with current featured articles.
• Zoom Dinosaurs (www.enchantedlearning.com) From Enchanted Learning. Kids’ site, info pages and stats, theories, history.
• Dinosaur genus list contains data tables on nearly every published Mesozoic dinosaur genus as of January 2011.
• LiveScience.com Giant Dinosaurs Get Downsized by LiveScience, June 21, 2009

Technical

• Palaeontologia Electronica From Coquina Press. Online technical journal.
• Dinobase A searchable dinosaur database, from the University of Bristol, with dinosaur lists, classification, pictures, and more.
• DinoData (www.dinodata.org) Technical site, essays, classification, anatomy.
• Dinosauria On-Line (www.dinosauria.com) Technical site, essays, pronunciation, dictionary.
• Thescelosaurus! By Justin Tweet. Includes a cladogram and small essays on each relevant genera and species.
• Dinosauromorpha Cladogram^{[dead link]} From Palaeos. A detailed amateur site about all things paleo.