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Dinosaur

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Birds are avian dinosaurs, and phylogenetic taxonomy includes over 11,000 extant species in the group Dinosauria.

Dinosaurs are a diverse group of reptilesTemplate:Refn of the clade Dinosauria. They first appeared during the Triassic period, between 243 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 11,000 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 most 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 Template:Convert and heights of Template:Convert 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 Template:Convert 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". 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 the Jurassic Park franchise. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.

DefinitionEdit

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.<ref name=MJB04dino/> 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.<ref name=olshevsky2000/> Both definitions cover the same known genera: Dinosauria = Ornithischia + Saurischia. This 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).<ref name="Letal05"/>

Birds are the sole surviving dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, most contemporary paleontologists reject the traditional style of classification based on anatomical similarity, in favor of phylogenetic taxonomy based on deduced ancestry, in which each group is defined as all descendants of a given founding genus.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Birds belong to the dinosaur subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians.<ref name=KP04/>

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. This would cause sauropods and kin to fall outside traditional dinosaurs, so they re-defined Dinosauria 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. They also resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda.<ref name="NYT-20170322">Template:Cite news "A version of this article appears in print on March 28, 2017, on Page D6 of the New York edition with the headline: Shaking Up the Dinosaur Family Tree."</ref><ref name="NAT-20170322">Template:Cite journal</ref>

General descriptionEdit

File:LA-Triceratops mount-2.jpg
Triceratops skeleton, Natural History Museum of Los Angeles County

Using one of the above definitions, dinosaurs can be generally described as archosaurs with hind limbs held erect beneath the body.<ref name=DFG97/> Other prehistoric animals, including pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs. 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.<ref name=DL90/>

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.<ref name=MM97/> Dinosaurs have always been recognized as an extremely varied group: over 900 non-avian dinosaur genera have been confidently identified (2018) with 1124 species (2016). Estimates put the total number of dinosaur genera preserved in the fossil record at 1850, nearly 75% still undiscovered,<ref name="Genera900">Template:Cite journal</ref><ref name="Howmany">Template:Cite journal</ref><ref name="Wang&Dodson"/> and the number that ever existed (in or out of the fossil record) at 3,400.<ref name=russell1995/> A 2016 estimate put the number of dinosaur species living in the Mesozoic at 1,543–2,468,<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> compared to the number of modern-day birds (avian dinosaurs) at 10,806 species.<ref>Template:Cite journal</ref>

Extinct dinosaurs, as well as modern birds, include genera that are herbivorous and others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. While dinosaurs were ancestrally bipedal (as are all modern birds), some evolved into quadrupeds, and others, such as Anchisaurus and Iguanodon, could walk 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 the best-known genera are remarkable for their 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 the Early Jurassic epoch at latest.<ref name=MacLeod>Template:Cite journal</ref> 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.<ref name=theropods/>

Distinguishing anatomical featuresEdit

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.<ref name="B012"/>

File:Dromaeosaurus skull en.svg
Labeled diagram of a typical archosaur skull, the skull of Dromaeosaurus

A detailed assessment of archosaur interrelations by Sterling Nesbitt<ref name=nesbitt2011/> 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.<ref name=MJB04dino/>

File:Sprawling and erect hip joints - horizontal.svg
Hip joints and hindlimb postures of: (left to right) typical reptiles (sprawling), dinosaurs and mammals (erect), and rauisuchians (pillar-erect)

A variety of other skeletal features are shared by dinosaurs. However, because they either are 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.<ref name=TRHJ00/> 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);<ref name=MJB04dino/> and a perforate acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia tupiniquim, for example).<ref name="UC Berkeley Journal of Earth Sciences">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=LARB99/> 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.<ref name=NIP07/>

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.<ref name=Holland1909/> 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.<ref name=MJB00/> Their erect posture enabled early dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.<ref name=RC05/> Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.<ref name=TKMB07/> 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.<ref name=TKMB07/>

History of studyEdit

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Pre-scientific historyEdit

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 (Template:Zh), a gazetteer compiled by Chang Qu (Template:Zh) during the Western Jin Dynasty (265–316), reported the discovery of dragon bones at Wucheng in Sichuan Province.<ref name=dong1992/> Villagers in central China have long unearthed fossilized "dragon bones" for use in traditional medicines.<ref name=BBCdinobonemed/> In Europe, dinosaur fossils were generally believed to be the remains of giants and other biblical creatures.<ref name=benton2000>Template:Harvnb</ref>

Early dinosaur researchEdit

File:William Buckland c1845.jpg
William Buckland

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,<ref name=WAS97/> 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).<ref>Template:Harvnb</ref> 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.<ref>Template:Harvnb</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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. In it he described and named a sauropod tooth, "Rutellum impicatum",<ref name=L99/><ref name=DS02/> that had been found in Caswell, near Witney, Oxfordshire.<ref name=G45/>

File:Dinosaur coining of the word in 1841.jpg
Sir Richard Owen's coining of the word dinosaur, in the 1842 revised version of his talk at an 1841 meeting of the British Association for the Advancement of Science.

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.<ref name=WAS97/><ref name=buckland1824/> The second non-avian dinosaur genus to be identified, Iguanodon, was purportedly discovered in 1822 by Mary Ann Mantell, the wife of English geologist Gideon Mantell, though this is disputed and some historians say Gideon had acquired remains years earlier. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas and published his findings in 1825.<ref name=GM25/><ref name=HDS97/>

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.<ref name=":2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":3">Template:Cite news</ref><ref name=":4">Template:Cite book</ref><ref name=Owen1841/><ref>Template:Cite Merriam-Webster</ref> The term is derived Template:Ety.<ref name=Owen1841/><ref name=LSJ/> Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it also to evoke their size and majesty.<ref name=FBS97/> Owen recognized that the remains that had been found so far, Iguanodon, Megalosaurus and Hylaeosaurus, shared 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 introduced the Dinosauria only in the revised text version of his talk published in April 1842.<ref name=":2" /><ref name=":3" /> 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.<ref name=owen94>Template:Harvnb</ref>

Discoveries in North AmericaEdit

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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.<ref name=weishampel06>Template:Cite journal</ref>

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.<ref name=Holmes/>

"Dinosaur renaissance" and beyondEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} 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.<ref name="taylor2010">Template:Cite journal</ref><ref name="Naish">Template:Cite book</ref> At the end of the 1960s, however, the field of dinosaur research experienced a surge in activity that remains ongoing.<ref name="arbour2018">Template:Cite journal</ref> 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.<ref name="taylor2010"/> Concurrently, Robert T. Bakker published a series of studies that likewise argued for active lifestyles in dinosaurs based on anatomical and ecological evidence (see Template:Section link),<ref name="bakker1968"/><ref name="bakker1972"/> which were subsequently summarized in his 1986 book The Dinosaur Heresies.<ref name=bakker86>Template:Harvnb</ref>

File:Dr. Bob Bakker with Dino.jpg
Paleontologist Robert T. Bakker with a mounted skeleton of a tyrannosaurid (Gorgosaurus libratus)

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.<ref name="Genera900"/> In 2008 over 30 new species of dinosaurs were named each year.<ref name="benton2008"/> 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.<ref name="cashmore2020">Template:Cite journal</ref> 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.<ref name="benton2008">Template:Cite journal</ref> Asian localities have produced the most complete theropod specimens,<ref name="cashmore2019">Template:Cite journal</ref> while North American localities have produced the most complete sauropodomorph specimens.<ref name="cashmore2020"/>

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.<ref name="holtz1997">Template:Cite book</ref><ref name="NYT-20161208">Template:Cite news</ref> Reference books summarizing the state of dinosaur research, such as David B. Weishampel and colleagues' The Dinosauria, made knowledge more accessible<ref name="lockley2000">Template:Cite journal</ref> 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.<ref name="lloyd2008">Template:Cite journal</ref>

Soft tissue and molecular preservationEdit

File:Dakota skin impression.jpg
An Edmontosaurus specimen's skin impressions found in 1999

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.<ref name="schweitzer2011">Template:Cite journal</ref> 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.<ref name="hooley1917"/> 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.<ref name="osborn1912">Template:Cite journal</ref> Owen and Hooley subsequently described skin impressions of Hypsilophodon and Iguanodon in 1885 and 1917.<ref name="hooley1917">Template:Cite journal</ref> Since then, scale impressions have been most frequently found among hadrosaurids, where the impressions are known from nearly the entire body across multiple specimens.<ref name="bell2014">Template:Cite book</ref>

Template:Multiple image Starting from the 1990s, major discoveries of exceptionally preserved fossils in deposits known as conservation Lagerstätten contributed to research on dinosaur soft tissues.<ref name="eliason2017">Template:Cite journal</ref><ref name="benton1998">Template:Cite journal</ref> 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 Template:Section link) have been described by Xing Xu and colleagues.<ref name="zhou2017">Template:Cite journal</ref><ref name="norell2005">Template:Cite journal</ref> In living reptiles and mammals, pigment-storing cellular structures known as melanosomes are partially responsible for producing colouration.<ref name="roy2020"/><ref name="vinther2020">Template:Cite journal</ref> 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.<ref name="zhang2010">Template:Cite journal</ref> This has enabled multiple full-body reconstructions of dinosaur colouration, such as for Sinosauropteryx<ref name="smithwick2017">Template:Cite journal</ref> and Psittacosaurus<ref name="vinther2016">Template:Cite journal</ref> by Jakob Vinther and colleagues, and similar techniques have also been extended to dinosaur fossils from other localities.<ref name="roy2020">Template:Cite journal</ref> (However, some researchers have also suggested that fossilized melanosomes represent bacterial remains.<ref name="lindgren2015">Template:Cite journal</ref><ref name="schweitzer2015">Template:Cite journal</ref>) 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).<ref name="zhou2014">Template:Cite journal</ref><ref name="oconnor2019">Template:Cite journal</ref> 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.<ref name="softtissue"/>

File:9121 - Milano, Museo storia naturale - Scipionyx samniticus - Foto Giovanni Dall'Orto 22-Apr-2007a.jpg
Scipionyx fossil with intestines, Natural History Museum of Milan

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.<ref name="morell1993">Template:Cite journal</ref><ref name="pawlicki1996">Template:Cite journal</ref><ref name="pawlicki1998">Template:Cite journal</ref> 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.<ref name=Schweitzer2005/> This discovery suggested that original soft tissues could be preserved over geological time,<ref name="schweitzer2011"/> with multiple mechanisms having been proposed.<ref name="anderson2023">Template:Cite journal</ref> 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.<ref name="schweitzer2009">Template:Cite journal</ref> Both specimens yielded collagen protein sequences that were viable for molecular phylogenetic analyses, which grouped them with birds as would be expected.<ref name="schweitzer2009"/><ref name="organ2008">Template:Cite journal</ref> The extraction of fragmentary DNA has also been reported for both of these fossils,<ref name="schweitzer2013">Template:Cite journal</ref> along with a specimen of Hypacrosaurus.<ref name="bailleul2020">Template:Cite journal</ref> 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.<ref name="bertazzo2015">Template:Cite journal</ref> Suggestions that these structures represent bacterial biofilms<ref name="kaye2008">Template:Cite journal</ref> have been rejected,<ref name="peterson2010">Template:Cite journal</ref> but cross-contamination remains a possibility that is difficult to detect.<ref name="buckley2017">Template:Cite journal</ref>

Evolutionary historyEdit

Origins and early evolutionEdit

File:Herrerasaurusskeleton.jpg
The early dinosaurs Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skull, from the Triassic

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.<ref name=KPA/><ref name=TannerLucas/> The oldest dinosaur fossils known from substantial remains date to the Carnian epoch of the Triassic period and have been found primarily in the Ischigualasto and Santa Maria Formations of Argentina and Brazil, and the Pebbly Arkose Formation of Zimbabwe.<ref name="griffin2022">Template:Cite journal</ref>

The Ischigualasto Formation (radiometrically dated at 231–230 million years old<ref name="desojo2020">Template:Cite journal</ref>) has produced the early saurischian Eoraptor, originally considered a member of the Herrerasauridae<ref name="OARM2010">Template:Cite journal</ref> but now considered to be an early sauropodomorph, along with the herrerasaurids Herrerasaurus and Sanjuansaurus, and the sauropodomorphs Chromogisaurus, Eodromaeus, and Panphagia.<ref name="novas2021">Template:Cite journal</ref> EoraptorTemplate:'s likely resemblance to the common ancestor of all dinosaurs suggests that the first dinosaurs would have been small, bipedal predators.<ref name="Daemonosaurus">Template:Cite journal</ref><ref name=Sereno1999/><ref name=SFRM93/> The Santa Maria Formation (radiometrically dated to be older, at 233.23 million years old<ref name=langer18>Template:Cite journal</ref>) has produced the herrerasaurids Gnathovorax and Staurikosaurus, along with the sauropodomorphs Bagualosaurus, Buriolestes, Guaibasaurus, Macrocollum, Nhandumirim, Pampadromaeus, Saturnalia, and Unaysaurus.<ref name="novas2021"/> 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.<ref name="griffin2022"/>

Less well-preserved remains of the sauropodomorphs Jaklapallisaurus and Nambalia, along with the early saurischian Alwalkeria, are known from the Upper Maleri and Lower Maleri Formations of India.<ref name="novas2011">Template:Cite journal</ref> 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.<ref name="novas2021"/><ref name="mariscano2016">Template:Cite journal</ref> Dinosaurs may have appeared as early as the Anisian epoch of the Triassic, approximately 243 million years ago, which is the age of Nyasasaurus from the Manda Formation of Tanzania. However, its known fossils are too fragmentary to identify it as a dinosaur or only a close relative.<ref name=nyasasaurus>Template:Cite journal</ref> 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<ref name="marsicano2015">Template:Cite journal</ref> to 21 million years.<ref name="langer18"/>

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.<ref>Template:Cite journal</ref> 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,<ref>Template:Harvnb</ref><ref>Template:Cite journal</ref> and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.<ref name=MJB04dino/> 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.<ref name="Letal05"/> Also notably, there was a heightened rate of extinction during the Carnian pluvial event.<ref>Template:Cite news</ref>

Evolution and paleobiogeographyEdit

File:Pangaea 200Ma.jpg
The supercontinent Pangaea in the early Mesozoic (around 200 million years ago).

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.<ref name=HCL04/> 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.<ref name=FS04/> 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.<ref name=HCL04/> 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.<ref name=FS04/> Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.<ref name=KP04/>

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.<ref name=HCL04/> 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.<ref name=FS04/> Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.<ref name=serenoetal07/>

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.<ref name=HCL04/> Flowering plants were greatly radiating,<ref name=FS04/> with the first grasses appearing by the end of the Cretaceous.<ref name=PSAS05/> 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.<ref name=FS04/>

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.<ref name=AF04/>

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.<ref name=lindow>Template:Harvnb</ref> Terror birds and bathornithids occupied carnivorous guilds alongside predatory mammals,<ref name="Cracraft">Template:Cite journal</ref><ref>Template:Cite journal</ref> and ratites are still fairly successful as midsized herbivores; eogruiids similarly lasted from the Eocene to Pliocene, becoming extinct only very recently after over 20 million years of co-existence with many mammal groups.<ref>Template:Harvnb</ref>

ClassificationEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Template:Multiple image

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.<ref name="B012"/>

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 {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}) meaning "lizard" and {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}) meaning "hip joint"—retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.<ref name=MJB00/> This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,<ref name=GSP88/> therizinosauroids,<ref name=clarketal2004/> dromaeosaurids,<ref name=MAPM04/> and birds<ref name=KP04/>). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups).<ref name=theropods/><ref name=sauropodomorphs>Template:Cite journal</ref>

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".<ref name="B012"/>

TaxonomyEdit

File:Ornithopods jconway.jpg
Restoration of six ornithopods; far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (large) Tenontosaurus.
File:Ceratopsidae-BW-003.png
Restoration of four ceratopsids: top left – Triceratops, top right – Styracosaurus, bottom left – Anchiceratops, bottom right – Chasmosaurus.
File:Macronaria scrubbed enh.jpg
Restoration of four macronarian sauropods: from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus
File:Dromaeosaurs.png
Restoration of six dromaeosaurid theropods: from left to right Microraptor, Velociraptor, Austroraptor, Dromaeosaurus, Utahraptor, and Deinonychus

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Further details and other hypotheses of classification may be found on individual articles.

Timeline of major groupsEdit

Timeline of major dinosaur groups per Template:Harvcoltxt.

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 id:cretaceous   value:rgb(0.5,0.78,0.31)
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 id:paleocene     value:rgb(0.99,0.65,0.37) 
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 id:miocene     value:rgb(0.999999,0.999999,0) 
 id:pliocene     value:rgb(0.97,0.98,0.68)  
 id:quaternary   value:rgb(0.98,0.98,0.5)
 id:pleistocene   value:rgb(0.999999,0.95,0.68)
 id:holocene   value:rgb(0.999,0.95,0.88)
 id:her   value:red Legend:Herrerasauria
 id:pur   value:purple Legend:Sauropodomorpha
 id:ther value:orange  Legend:Theropoda
 id:orn  value:green   Legend:Ornithischia

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bar:periodtop
from: -251   till:  -245    color:earlytriassic  text:Ear.
from: -245   till:  -228    color:middletriassic  text:Middle 
from: -228   till:  -199.6    color:latetriassic  text:Late            
from: -199.6    till: -175.6    color:earlyjurassic    text:Early
from: -175.6    till: -161.2    color:middlejurassic    text:Middle
from: -161.2    till: -145.5    color:latejurassic    text:Late
from: -145.5    till: -99.6    color:earlycretaceous    text:Early
from: -99.6    till: -65.5    color:latecretaceous    text:Late
from: -65.5   till:  -55.8    color:paleocene  text:Paleo.
from: -55.8   till:  -33.9    color:eocene  text:Eocene 
from: -33.9   till:  -23.03    color:oligocene  text:Oligo.            
from: -23.03    till: -5.332    color:miocene    text:Mio.
from: -5.332    till: -2.588    color:pliocene    text:Pl.
from: -2.588    till: -0.0117   color:pleistocene    text:Pl.
from: -0.0117    till: 0    color:holocene    text:H.

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from: -251   till:  -199.6    color:triassic  text:Triassic         
from: -199.6    till: -145.5    color:jurassic    text:Jurassic
from: -145.5    till: -65.5   color:cretaceous    text:Cretaceous
from: -65.5   till:  -23.03    color:paleogene  text:Paleogene         
from: -23.03    till: -2.588    color:neogene    text:Neogene
from: -2.588    till: 0   color:quaternary    text:Q.

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align:left fontsize:M mark:(line,white) width:5 anchor:till shift:(5,-4)

color:her bar:NAM1  from:-233.23    till:-210 text:Herrerasauridae
color:pur bar:NAM2 from:-231.4 till:-208 text:Guaibasauridae
color:pur bar:NAM3 from:-225 till:-190 text:Plateosauridae
color:pur bar:NAM4 from:-228 till:-213 text:Riojasauridae
color:pur bar:NAM5 from:-227 till:-176 text:Massospondylidae
color:pur bar:NAM6 from:-183 till:-175 text:Vulcanodontidae
color:pur bar:NAM7 from:-168 till:-125 text:Turiasauria
color:pur bar:NAM8 from:-175 till:-150 text:Cetiosauridae
color:pur bar:NAM9 from:-174 till:-93 text:Diplodocoidea
color:pur bar:NAM10 from:-157 till:-93 text:Brachiosauridae
color:pur bar:NAM11 from:-140 till:-66 text:Titanosauria
color:ther bar:NAM12 from:-221 till:-183 text:Coelophysoidea
color:ther bar:NAM13 from:-199.3 till:-66 text:Ceratosauria
color:ther bar:NAM14 from:-170 till:-85 text:Megalosauroidea
color:ther bar:NAM15 from:-175.6 till:-88 text:Carnosauria
color:ther bar:NAM16 from:-130 till:-66 text:Megaraptora
color:ther bar:NAM17 from:-166 till:-66 text:Tyrannosauroidea
color:ther bar:NAM18 from:-151.5 till:-108 text:Compsognathidae
color:ther bar:NAM19 from:-140 till:-66 text:Ornithomimosauria
color:ther bar:NAM20 from:-160 till:-66 text:Alvarezsauria
color:ther bar:NAM21 from:-139 till:-66 text:Therizinosauria
color:ther bar:NAM22 from:-130 till:-66 text:Oviraptorosauria
color:ther bar:NAM23 from:-167 till:-66 text:Deinonychosauria
color:ther bar:NAM24 from:-155 till:0 shift:(-45,5) text:Avialae
color:orn bar:NAM25 from:-200 till:-140 text:Heterodontosauridae
color:orn bar:NAM26 from:-169 till:-100 text:Stegosauria
color:orn bar:NAM27 from:-170.3 till:-66 text:Ankylosauria
color:orn bar:NAM28 from:-92 till:-66 text:Pachycephalosauria
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from: -245   till:  -228    color:middletriassic  text:Middle 
from: -228   till:  -199.6    color:latetriassic  text:Late            
from: -199.6    till: -175.6    color:earlyjurassic    text:Early
from: -175.6    till: -161.2    color:middlejurassic    text:Middle
from: -161.2    till: -145.5    color:latejurassic    text:Late
from: -145.5    till: -99.6    color:earlycretaceous    text:Early
from: -99.6    till: -65.5    color:latecretaceous    text:Late
from: -65.5   till:  -55.8    color:paleocene  text:Paleo.
from: -55.8   till:  -33.9    color:eocene  text:Eocene 
from: -33.9   till:  -23.03    color:oligocene  text:Oligo.            
from: -23.03    till: -5.332    color:miocene    text:Mio.
from: -5.332    till: -2.588    color:pliocene    text:Pl.
from: -2.588    till: -0.0117   color:pleistocene    text:Pl.
from: -0.0117    till: 0    color:holocene    text:H.

bar:era
from: -251   till:  -199.6    color:triassic  text:Triassic         
from: -199.6    till: -145.5    color:jurassic    text:Jurassic
from: -145.5    till: -65.5   color:cretaceous    text:Cretaceous
from: -65.5   till:  -23.03    color:paleogene  text:Paleogene         
from: -23.03    till: -2.588    color:neogene    text:Neogene
from: -2.588    till: 0   color:quaternary    text:Q.

</timeline>

PaleobiologyEdit

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.<ref name="softtissue">Template:Cite journal</ref><ref name="Schweitzer2005"/> 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).<ref name=alexander2006/><ref name=dinobiology>Template:Cite journal</ref> Two topics of particular interest and study have been dinosaur size and behavior.<ref>Template:Harvnb</ref>

SizeEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

File:Longest dinosaur by clade.svg
Scale diagram comparing the average human to the longest known dinosaurs in five major clades:Template:LegendTemplate:LegendTemplate:Legend Template:Legend Template:Legend

Current evidence suggests that dinosaur average size varied through the Triassic, Early Jurassic, Late Jurassic and Cretaceous.<ref name=Sereno1999/> Predatory theropod dinosaurs, which occupied most terrestrial carnivore niches during the Mesozoic, most often fall into the Template:Convert category when sorted by estimated weight into categories based on order of magnitude, whereas recent predatory carnivoran mammals peak in the Template:Convert category.<ref name=JF93/> The mode of Mesozoic dinosaur body masses is between Template:Convert.<ref name=Peczkis1994/> This contrasts sharply with the average size of Cenozoic mammals, estimated by the National Museum of Natural History as about Template:Convert.<ref name=NMNH/>

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.<ref name="sander2011">Template:Cite journal</ref> 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.<ref name=KC06/>

Largest and smallestEdit

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 non-avian dinosaur 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.<ref>Template:Harvnb</ref>

File:Argentinosaurus 9.svg
Comparative size of Argentinosaurus to the average human

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;<ref name=EC68/> this mount is Template:Convert tall and Template:Convert long,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and would have belonged to an animal that weighed between Template:Gaps and Template:Gaps kilograms (Template:Gaps and Template:Gaps lb). The longest complete dinosaur is the Template:Convert long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Museum of Natural History in 1907.<ref name=lucas04>Template:Cite conference</ref> The longest dinosaur known from good fossil material is Patagotitan: the skeleton mount in the American Museum of Natural History in New York is Template:Convert long. The Museo Municipal Carmen Funes in Plaza Huincul, Argentina, has an Argentinosaurus reconstructed skeleton mount that is Template:Convert long.<ref name="PLOS One">Template:Cite journal</ref>

File:Maraapunisaurus Skeletal V1.svg
Maraapunisaurus, one of the largest animals to walk the earth.
File:Bruhathkayosaurus matleyi updated.png
Bruhathkayosaurus, potentially the largest terrestrial animal to ever exist.

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 Template:Convert and reached lengths of Template:Convert; some of the longest were the Template:Convert long Diplodocus hallorum<ref name=KC06/> (formerly Seismosaurus), the Template:Convert long Supersaurus,<ref name=LHW07/> and Template:Convert long Patagotitan; and the tallest, the Template:Convert tall Sauroposeidon, which could have reached a sixth-floor window. There were a few dinosaurs that was considered either the heaviest and longest. The most famous one include 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 Template:Convert long and weighed Template:Cvt.<ref name=KC06/> However, recent research have placed Amphicoelias from the long, gracile diplodocid to the shorter but much stockier rebbachisaurid. Now renamed as Maraapunisaurus, this sauropod now stands as much as Template:Convert long and weigh as much as Template:Cvt.<ref name="carpenter2018">Template:Cite journal</ref><ref>Template:Cite journal</ref> Another contender of this title includes Bruhathkayosaurus, a controversial taxon that was recently confirmed to exist after archived photos were uncovered.<ref>Template:Cite journal</ref> Bruhathkayosaurus was a titanosaur and would have most likely weighed more than even Marrapunisaurus. Recent size estimates in 2023 have placed this sauropod reaching lengths of up to Template:Cvt long and a colossal weight range of around Template:Cvt, if these upper estimates up true, Bruhathkayosaurus would have rivaled the blue whale and Perucetus colossus as one of the largest animals to have ever existed.<ref name="Bruhathkayosaurus2023">Template:Cite journal</ref>

The largest carnivorous dinosaur was Spinosaurus, reaching a length of Template:Convert and weighing Template:Convert.<ref name=SMBM06/><ref name=TH07/> Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus, and Tyrannosaurus.<ref name=TH07/> Therizinosaurus and Deinocheirus were among the tallest of the theropods. The largest ornithischian dinosaur was probably the hadrosaurid Shantungosaurus giganteus which measured Template:Convert.<ref>Template:Cite journal</ref> The largest individuals may have weighed as much as Template:Convert.<ref>Template:Harvnb</ref>

File:Bee hummingbird (Mellisuga helenae) adult male in flight-cropped.jpg
An adult bee hummingbird, the smallest known dinosaur

The smallest dinosaur known is the bee hummingbird,<ref>Template:Harvnb</ref> with a length of only Template:Convert and mass of around Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds.<ref name=zhang2008/> For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of Template:Convert<ref name=anchiadvance/> and a total skeletal length of Template:Convert.<ref name=zhang2008/><ref name=anchiadvance/> The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about Template:Convert long each.<ref name=Holtz2007/><ref name="butler&zhao2009"/>

BehaviorEdit

File:Maiasaurusnest.jpg
A nesting ground of the hadrosaur Maiasaura peeblesorum was discovered in 1978

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 crocodilians (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.<ref name=alexander2006/>

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.<ref name=Yans/> 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,<ref name=Day2002/> although there is no evidence for specific herd structures.<ref name=JLW05/> 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.<ref name=DVetal08sino/> The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.<ref name=LG93/><ref name="maxwell&ostrom1995"/> 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.<ref name=RB07/>

File:Centrosaurus dinosaur.png
Restoration of two Centrosaurus apertus engaged in intra-specific combat

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.<ref name=PC98/>

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,<ref name=AMNH/> providing evidence that dinosaurs did indeed attack each other.<ref name=carpenter1998/> 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.<ref name=carpenter1998/> Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.<ref name=rogersetal2003/>

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.<ref name=SchmitzMotani2011/>

Based on fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle.<ref name=VMK07/> Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines.<ref>Template:Harvnb</ref> While some early bird-like species may have already been arboreal as well (including dromaeosaurids) such as Microraptor<ref name=chatterjee2007/>) 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,<ref name=alexander2006/> whether diplodocids could create sonic booms via whip-like tail snapping,<ref name=goriely/> and whether sauropods could float.<ref name=Henderson2006/>

CommunicationEdit

Modern birds communicate by 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.<ref name=senter2008/> Reconstruction of the plumage color of Anchiornis suggest the importance of color in visual communication in non-avian dinosaurs.<ref name="Li2010"/> Vocalization in non-avian dinosaurs is less certain. In birds, the larynx plays no role in sound production. Instead, birds vocalize with a novel organ, the syrinx, farther down the trachea.<ref name=":22"/> The earliest remains of a syrinx were 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.<ref name="Clarke2016"/>

File:Lambeosaurus magnicristatus DB.jpg
Restoration of a striking and unusual visual display in a Lambeosaurus magnicristatus. The crest may also have acted as a resonating chamber for sounds.

A 2009 review indicated that non-avians used visual displays and possibly non-vocal sounds, such as hissing, jaw-grinding or -clapping, splashing, and wing-beating (possible in winged maniraptoran dinosaurs).<ref name=senter2008/> 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, without a "silent period" in bird evolution.<ref name=":0" />

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; but the mobile cricoid–arytenoid joint and long arytenoid cartilages would have allowed 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.<ref name=":1" /> A 2016 study concludes that some dinosaurs may have produced closed-mouth vocalizations, such as 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.<ref name="Tobias2016"/> The crests of some hadrosaurids and the nasal chambers of ankylosaurids may have been resonators.<ref name="Weishampel981"/><ref name="Witmer"/>

Reproductive biologyEdit

Template:See also

File:Gniazdo sieweczki RB.JPG
Nest of a plover (Charadrius)

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.<ref name="Hansell">Template:Harvnb</ref> 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.<ref name="Varrichioetal.02">Template:Cite journal</ref>

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.<ref name=LW08/>

File:Citipati AMNH.jpg
Fossil interpreted as a nesting oviraptorid Citipati at the American Museum of Natural History.

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.<ref name=HM79/> A specimen of the oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993,<ref name="search.eb"/> which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.<ref>Template:Harvnb</ref> An embryo of the basal sauropodomorph Massospondylus was found without teeth, indicating that some parental care was required to feed the young dinosaurs.<ref name=Reiszetal05/> Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.<ref name=BBCtracks/>

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Both Tyrannosaurus and Troodon had juveniles with clear superprecociality and likely occupying different ecological niches than the adults.<ref name="Varrichioetal.02"/> Superprecociality has been inferred for sauropods.<ref>Template:Cite journal</ref>

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.<ref name="cloacal-vent">Template:Cite journal</ref>

PhysiologyEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} 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.<ref name=CH04/> 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.<ref name="pontzer2009">Template:Cite journal</ref> 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,<ref name="benson2018">Template:Cite journal</ref> which is termed mesothermy by some.<ref name="grady2014">Template:Cite journal</ref> 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.<ref name="legendre2016">Template:Cite journal</ref><ref name="seymour2004">Template:Cite journal</ref>

File:Pasta-Brontosaurus.jpg
This 1897 restoration of Brontosaurus as an aquatic, tail-dragging animal, by Charles R. Knight, typified early views on dinosaur lifestyles.

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.<ref name="bakker1968">Template:Cite journal</ref><ref name="bakker1972">Template:Cite journal</ref><ref name="taylor2010"/><ref name="parsons2001"/>

One of the greatest contributions to the modern understanding of dinosaur physiology has been paleohistology, the study of microscopic tissue structure in dinosaurs.<ref name="erickson2014">Template:Cite journal</ref><ref name="bailleul2019">Template:Cite journal</ref> 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,<ref name="dericqles1974">Template:Cite journal</ref><ref name="dericqles1980">Template:Cite book</ref> though not universally present.<ref name="padian2004">Template:Cite journal</ref><ref name="desouza2020">Template:Cite journal</ref> This has led to a significant body of work in reconstructing growth curves and modeling the evolution of growth rates across various dinosaur lineages,<ref name="physiologyrefs">For examples of this work conducted on different dinosaur lineages, see

File:Dino bird h.jpg
Comparison between the air sacs of an abelisaur and a bird

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.<ref name=Sereno2008/> Such respiratory systems, which may have appeared in the earliest saurischians,<ref name="oconnor2009">Template:Cite journal</ref> 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.<ref name="sander2011"/> The rapid airflow would also have been an effective cooling mechanism, which in conjunction with a lower metabolic rate<ref name="eagle2011">Template:Cite journal</ref> would have prevented large sauropods from overheating. These traits may have enabled sauropods to grow quickly to gigantic sizes.<ref name="wedel2003">Template:Cite journal</ref><ref name="perry2009">Template:Cite journal</ref> 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.<ref name="sander2011"/><ref name="alexander1998">Template:Cite journal</ref>

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.<ref name="benson2018"/> In most living species, uric acid is excreted along with feces as a semisolid waste.<ref name="tsahar2005">Template:Cite journal</ref><ref name="skadhauge2003">Template:Cite journal</ref> However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.<ref name="preest1997">Template:Cite journal</ref> This material, as well as the output of the intestines, emerges from the cloaca.<ref name="mora1965">Template:Cite journal</ref><ref name="packard1966">Template:Cite journal</ref> In addition, many species regurgitate pellets,<ref name="balgooyen1971">Template:Cite journal</ref> and fossil pellets are known as early as the Jurassic from Anchiornis.<ref name="xu2018">Template:Cite journal</ref>

Template:AnchorThe 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.<ref name="russell1997"/> 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.<ref name="brusatte2012"/>

Origin of birdsEdit

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The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley.<ref name=huxley1868/> 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.<ref name=heilmann/> However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;<ref name=KP04/> they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.<ref name=HO24/> In the 1970s, John Ostrom revived the dinosaur–bird theory,<ref name=ostrom1973/> which gained momentum in the coming decades with the advent of cladistic analysis,<ref name=gauthier1986/> and a great increase in the discovery of small theropods and early birds.<ref name=TRHJ00/> 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.<ref name="NYT-20161208" /><ref name=KP04/> Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.<ref name=Mayretal2005/> They are most closely allied with maniraptoran coelurosaurs.<ref name=KP04/> 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,<ref name=martin2004/> or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.<ref name=AF02/>

FeathersEdit

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File:Feathered non-avian Maniraptora.jpg
Various feathered non-avian dinosaurs, including Archaeopteryx, Anchiornis, Microraptor and Zhenyuanlong

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.<ref name=switeknature>Template:Cite journal</ref> Direct fossil evidence of feathers or feather-like structures has been discovered in a diverse array of species in many non-avian dinosaur groups,<ref name="NYT-20161208" /> both among saurischians and ornithischians. Simple, branched, feather-like structures are known from heterodontosaurids, primitive neornithischians,<ref name="Godefroit2014">Template:Cite journal</ref> and theropods,<ref name=Xuetal2004/> 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.<ref name=KP04/><ref name=GC06/> Feather-like structures known as pycnofibres have also been found in pterosaurs.<ref name=kellneretal2009>Template:Cite journal</ref>

However, researchers do not agree regarding whether these structures share a common origin between lineages (i.e., they are homologous),<ref name="mayr2016">Template:Cite journal</ref><ref name="benton2019"/> or if they were the result of widespread experimentation with skin coverings among ornithodirans.<ref name="barrett2015">Template:Cite journal</ref> If the former is the case, filaments may have been common in the ornithodiran lineage and evolved before the appearance of dinosaurs themselves.<ref name=switeknature/> 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.<ref name=AlibL2006>Template:Cite journal</ref> 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,<ref name=TLS03/><ref name=FLH05/><ref name=LSFX07/> and that maniraptoran dinosaurs with vaned feathers were not actually dinosaurs, but convergent with dinosaurs.<ref name=AF02/><ref name=FLH05/> 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.<ref name=Prum2003/>

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.<ref name=PW88/> 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.<ref name="benton2019">Template:Cite journal</ref> 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.<ref name="Schweitzeretal1999">Template:Cite journal</ref>

SkeletonEdit

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.<ref name=archaeopteryxucmp>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Soft anatomyEdit

File:Pneumatopores on the left ilium of the theropod Aerosteon riocoloradensis.jpg
Pneumatopores on the left ilium of Aerosteon riocoloradensis

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.<ref name=OConnorClaessens2005/><ref>Template:Cite journal</ref> 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.<ref name=Sereno2008/><ref name=newswise2/>

Behavioral evidenceEdit

Fossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.<ref name=XUNorell2004/> 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.<ref name="norell1995">Template:Cite journal</ref> 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.<ref name=Varricchioetal2008>Template:Cite journal</ref>

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.<ref name=wings2007/>

Extinction of major groupsEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} All non-avian dinosaurs and most lineages of birds<ref name="longrich2011">Template:Cite journal</ref> 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,<ref name="renne2013">Template:Cite journal</ref> 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.<ref name="brusatte2014">Template:Cite journal</ref> 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.<ref name="macleod1997"/><ref name="archibald1982"/> In all, approximately 47% of genera and 76% of species on Earth became extinct during the K-Pg extinction event.<ref name="jablonski1991">Template:Cite journal</ref> 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;<ref>Template:Cite journal</ref> 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.<ref name="field2018">Template:Cite journal</ref><ref name="larson2016">Template:Cite journal</ref>

Pre-extinction diversityEdit

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.<ref name="leloeuff2012">Template:Cite journal</ref> 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.<ref name="macleod1997"/><ref name="archibald1982">Template:Cite journal</ref><ref name="carpenter1983">Template:Cite journal</ref> However, these comparisons did not account either for varying preservation potential between rock units or for different extents of exploration and excavation.<ref name="brusatte2014"/> In 1984, Dale Russell carried out an analysis to account for these biases, and found no evidence of a decline;<ref name="russell1984">Template:Cite journal</ref> another analysis by David Fastovsky and colleagues in 2004 even showed that dinosaur diversity continually increased until the extinction,<ref name="fastovsky2004">Template:Cite journal</ref> but this analysis has been rebutted.<ref name="sullivan2006">Template:Cite book</ref> Since then, different approaches based on statistics and mathematical models have variously supported either a sudden extinction<ref name="brusatte2014"/><ref name="leloeuff2012"/><ref name="chiarenza2019">Template:Cite journal</ref> or a gradual decline.<ref name="lloyd2011">Template:Cite journal</ref><ref name="sakamoto2016">Template:Cite journal</ref> 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.<ref name="barrett2009">Template:Cite journal</ref><ref name="upchurch2011">Template:Cite journal</ref>

Impact eventEdit

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File:LWA with Walt.JPG
Luis (left) and his son Walter Alvarez (right) at the K-T Boundary in Gubbio, Italy, 1981
File:Chicxulub radar topography.jpg
The Chicxulub Crater at the tip of the Yucatán Peninsula; the impactor that formed this crater may have caused the dinosaur extinction.

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.<ref>Template:Harvnb</ref> 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.<ref name="alvarez1980"/> Shocked quartz, indicative of a strong shockwave emanating from an impact, was also found worldwide.<ref name="bohor1987">Template:Cite journal</ref> The actual impact site remained elusive until a crater measuring Template:Convert wide was discovered in the Yucatán Peninsula of southeastern Mexico, and was publicized in a 1991 paper by Alan Hildebrand and colleagues.<ref name="hildebrand1991">Template:Cite journal</ref> Now, the bulk of the evidence suggests that a bolide Template:Convert wide impacted the Yucatán Peninsula 66 million years ago, forming this crater<ref name="pope1996"/> and creating a "kill mechanism" that triggered the extinction event.<ref name="schulte2010">Template:Cite journal</ref><ref name="kring2007">Template:Cite journal</ref><ref name="chiarenza2020">Template:Cite journal</ref>

Within hours, the Chicxulub impact would have created immediate effects such as earthquakes,<ref name="ivanov2005">Template:Cite journal</ref> tsunamis,<ref name="matsui2002">Template:Cite journal</ref> and a global firestorm that likely killed unsheltered animals and started wildfires.<ref name="robertson2004">Template:Cite journal</ref><ref name="robertson2013">Template:Cite journal</ref> However, it would also have had longer-term consequences for the environment. Within days, sulfate aerosols released from rocks at the impact site would have contributed to acid rain and ocean acidification.<ref name="pope1997">Template:Cite journal</ref><ref name="ohno2014"/> 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.<ref name="brusatte2014"/><ref name="kaiho2016">Template:Cite journal</ref><ref name="lyons2020">Template:Cite journal</ref> (This role was ascribed to sulfate aerosols until experiments demonstrated otherwise.<ref name="ohno2014">Template:Cite journal</ref>) 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.<ref name="larson2016"/>

Deccan TrapsEdit

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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.<ref name="chenet2009">Template:Cite journal</ref><ref name="schoene2019">Template:Cite journal</ref> Greenhouse gases such as carbon dioxide and sulfur dioxide would have been released by this volcanic activity,<ref name="mclean1985">Template:Cite journal</ref><ref name="self2006">Template:Cite journal</ref> resulting in climate change through temperature perturbations of roughly Template:Convert but possibly as high as Template:Convert.<ref name="tobin2017">Template:Cite journal</ref> Like the Chicxulub impact, the eruptions may also have released sulfate aerosols, which would have caused acid rain and global cooling.<ref name="schmidt2016">Template:Cite journal</ref> 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.<ref name="renne2013"/><ref name="brusatte2014"/><ref name="hofman2000"/>

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;<ref name="mclean1985"/><ref name="sahni1988">Template:Cite journal</ref> even after the crater's discovery, the impact was still thought to only have had a regional, not global, effect on the extinction event.<ref name="glasby1996">Template:Cite journal</ref> In response, Luis Alvarez rejected volcanic activity as an explanation for the iridium layer and the extinction as a whole.<ref name="alvarez1987">Template:Cite report</ref> 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.<ref name="alvarez1997"/> Some estimates have placed the start of the second phase in the Deccan Traps eruptions within 50,000 years after the Chicxulub impact.<ref name="renne2015">Template:Cite journal</ref> 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.<ref name="richards2015">Template:Cite journal</ref><ref name="khazins2019">Template:Cite book</ref>

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 sulfur 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.<ref name="archibald2010">Template:Cite journal</ref><ref name="courtillot2010">Template:Cite journal</ref> They consider the Chicxulub impact to have worsened the ongoing climate change caused by the eruptions.<ref name="keller2014">Template:Cite journal</ref> 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.<ref name="schulte2010reply">Template:Cite journal</ref> 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.<ref name="chiarenza2020"/>

Possible Paleocene survivorsEdit

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.<ref name="fassett2011"/> If the bone was not re-deposited by weathering action, it would provide evidence that some dinosaur populations survived at least half a million years into the Cenozoic.<ref name="fassett2009"/> Other evidence includes the presence of dinosaur remains in the Hell Creek Formation up to Template:Convert 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.<ref name="sloan1986"/> However, these supposed Paleocene dinosaurs are considered by many other researchers to be reworked, that is, washed out of their original locations and then reburied in younger sediments.<ref name="lucas2009">Template:Cite journal</ref><ref name="renne2012">Template:Cite journal</ref><ref name="lofgren1990">Template:Cite journal</ref> The age estimates have also been considered unreliable.<ref name="koenig2012">Template:Cite journal</ref>

Cultural depictionsEdit

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File:Iguanodon Crystal Palace.jpg
Outdated Iguanodon statues created by Benjamin Waterhouse Hawkins for the Crystal Palace Park in 1853
File:Winsor McCay (1914)Gertie the Dinosaur.webm
Gertie the Dinosaur (1914) by Winsor McCay, featuring the first animated dinosaur

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.<ref name="m-w"/>

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.<ref name=torrens1993/> 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.<ref name=breithaupt1997/>

The popular preoccupation with dinosaurs has ensured their appearance in literature, film, and other media. Beginning in 1852 with a passing mention in Charles DickensTemplate:' Bleak House,<ref name=bleakhouse/> 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.<ref name=DFGlut1997/><ref>Template:Cite book</ref>

See alsoEdit

NotesEdit

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ReferencesEdit

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BibliographyEdit

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Further readingEdit

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