Editing Dinosaur (section)

==Definition==
Under [[phylogenetic nomenclature]], dinosaurs are usually defined as the group consisting of the [[most recent common ancestor]] (MRCA) of ''[[Triceratops]]'' and [[Bird#Diversification of modern birds|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 [[ankylosauria]]ns (armored herbivorous quadrupeds), [[stegosauria]]ns (plated herbivorous quadrupeds), [[ceratopsia]]ns (bipedal or quadrupedal herbivores with [[neck frill]]s), [[pachycephalosauria]]ns (bipedal herbivores with thick skulls), [[Ornithopoda|ornithopod]]s (bipedal or quadrupedal herbivores including "[[Hadrosauridae|duck-bills]]"), [[Theropoda|theropod]]s (mostly bipedal carnivores and birds), and [[sauropodomorpha|sauropodomorphs]] (mostly large herbivorous quadrupeds with long necks and tails).<ref name="Letal05"/>

Birds are the sole surviving dinosaurs. In traditional [[Taxonomy (biology)|taxonomy]], birds were considered a separate [[class (biology)|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 [[Phylogenetics|phylogenetic]] taxonomy based on deduced ancestry, in which each group is defined as all descendants of a given founding genus.<ref>{{cite web |url=https://evolution.berkeley.edu/evolibrary/article/evo_10 |url-status=live |title=Using the tree for classification |website=Understanding Evolution |publisher=[[University of California]] |location=[[Berkeley, California|Berkeley]] |archive-url=https://web.archive.org/web/20190831003846/https://evolution.berkeley.edu/evolibrary/article/evo_10 |archive-date=August 31, 2019 |access-date=October 14, 2019}}</ref> Birds belong to the dinosaur subgroup [[Maniraptora]], which are [[Coelurosauria|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'', ''[[house sparrow|Passer domesticus]]'' and ''[[Diplodocus|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">{{cite news |last=Wade |first=Nicholas |author-link=Nicholas Wade |title=Shaking Up the Dinosaur Family Tree |date=March 22, 2017 |url=https://www.nytimes.com/2017/03/22/science/dinosaur-family-tree.html |url-status=live |url-access=registration |work=[[The New York Times]] |location=[[New York City|New York]] |issn=0362-4331 |archive-url=https://web.archive.org/web/20180407234942/https://www.nytimes.com/2017/03/22/science/dinosaur-family-tree.html |archive-date=April 7, 2018 |access-date=October 30, 2019}} "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">{{cite journal |last1=Baron |first1=Matthew G. |last2=Norman |first2=David B. |author-link2=David B. Norman |last3=Barrett |first3=Paul M. |year=2017 |title=A new hypothesis of dinosaur relationships and early dinosaur evolution |journal=[[Nature (journal)|Nature]] |location=London |publisher=[[Nature Research]] |volume=543 |issue=7646 |pages=501–506 |bibcode=2017Natur.543..501B |doi=10.1038/nature21700 |issn=0028-0836 |pmid=28332513 |s2cid=205254710}}</ref>

===General description===
[[File:LA-Triceratops mount-2.jpg|thumb|upright=1.15|left|''[[Triceratops]]'' skeleton, [[Natural History Museum of Los Angeles County]]]]

Using one of the above definitions, dinosaurs can be generally described as [[archosaur]]s with [[Terrestrial locomotion#Posture|hind limbs held erect beneath the body]].<ref name=DFG97/> Other prehistoric animals, including [[pterosaur]]s, [[mosasaur]]s, [[ichthyosaur]]s, [[Plesiosauria|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 [[Avemetatarsalia|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; [[mammal]]s, 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">{{cite journal |last1=Tennant |first1=Jonathan P. |last2=Chiarenza |first2=Alfio Alessandro |last3=Baron |first3=Matthew |title=How has our knowledge of dinosaur diversity through geologic time changed through research history? |journal=PeerJ |date=19 February 2018 |volume=6 |pages=e4417 |doi=10.7717/peerj.4417|pmid=29479504 |pmc=5822849 |doi-access=free }}</ref><ref name="Howmany">{{cite journal |last1=Starrfelt |first1=Jostein |last2=Liow |first2=Lee Hsiang |title=How many dinosaur species were there? Fossil bias and true richness estimated using a Poisson sampling model |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=2016 |volume=371 |issue=1691 |article-number=20150219 |doi=10.1098/rstb.2015.0219|pmid=26977060 |pmc=4810813 }}</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>{{cite journal |last1=Starrfelt |first1=Jostein |last2=Liow |first2=Lee Hsiang |year=2016 |title=How many dinosaur species were there? Fossil bias and true richness estimated using a Poisson sampling model |journal=[[Philosophical Transactions of the Royal Society B]] |location=London |publisher=[[Royal Society]] |volume=371 |issue=1691 |article-number=20150219 |doi=10.1098/rstb.2015.0219 |issn=0962-8436 |pmc=4810813 |pmid=26977060}}</ref><ref>{{cite web |url=https://www.nationalgeographic.com/science/article/most-dinosaur-species-are-still-undiscovered/|url-status=live |url-access=registration |last=Black |first=Riley |title=Most Dinosaur Species Are Still Undiscovered |website=[[National Geographic News]] |date=March 23, 2016 |archive-url=https://web.archive.org/web/20210306214843/https://www.nationalgeographic.com/science/article/most-dinosaur-species-are-still-undiscovered/ |archive-date=March 6, 2021 |access-date=June 6, 2021}}</ref>  compared to the number of modern-day birds (avian dinosaurs) at 10,806 species.<ref>{{cite journal|last1=Gill|first1=F.|last2=Donsker|first2=D.|last3=Rasmussen|first3=P.|title=Welcome|year=2021|journal=IOC World Bird List 11.1.|url=https://www.worldbirdnames.org/new/|archive-date=July 30, 2023|access-date=January 25, 2021|archive-url=https://web.archive.org/web/20230730175525/https://www.worldbirdnames.org/new/|url-status=live}}</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>{{cite journal |last1=MacLeod |first1=Norman |last2=Rawson |first2=Peter F. |last3=Forey |first3=Peter L. |last4=Banner |first4=FT |last5=Boudagher-Fadel |first5=MK |last6=Bown |first6=PR |last7=Burnett |first7=JA |last8=Chambers |first8=P |last9=Culver |first9=S |last10=Evans |first10=SE |last11=Jeffery |first11=C |last12=Kaminski |first12=MA |last13=Lord |first13=AR |last14=Milner |first14=AC |last15=Milner |first15=AR |last16=Morris |first16=N |last17=Owen |first17=E |last18=Rosen |first18=BR |last19=Smith |first19=AB |last20=Taylor |first20=PD |last21=Urquhart |first21=E |last22=Young |first22=JR |display-authors=3 |year=1997 |title=The Cretaceous–Tertiary biotic transition |journal=[[Journal of the Geological Society]] |location=London |publisher=[[Geological Society of London]] |volume=154 |issue=2 |pages=265–292 |doi=10.1144/gsjgs.154.2.0265|bibcode=1997JGSoc.154..265M |s2cid=129654916 |issn=0016-7649}}</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 [[spinosauridae|spinosaurids]], had [[semiaquatic]] habits.<ref name=theropods/>

===Distinguishing anatomical features===
While recent discoveries have made it more difficult to present a universally agreed-upon list of their distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton, or are clearly descendants of older dinosaurs showing these modifications. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria; the earliest dinosaurs had them and passed them on to their descendants. Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the [[Synapomorphy and apomorphy|synapomorphies]] of such a group.<ref name="B012"/>
[[File:Dromaeosaurus skull en.svg|thumb|right|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 [[Skull#Structure|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 [[vertebra]]e behind the [[Atlas (anatomy)|atlas]] and [[Axis (anatomy)|axis]], the first two neck vertebrae
* Apex of a deltopectoral crest (a projection on which the [[clavipectoral triangle|deltopectoral]] muscles attach) located at or more than 30% down the length of the [[humerus]] (upper arm bone)
* [[Radius (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 [[Talus bone|astragalus]] and [[Calcaneus|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 (bone)|ilium]] and the [[Pubis (bone)|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 [[Silesauridae|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|thumb|Hip joints and hindlimb postures of: (left to right) typical [[reptile]]s (sprawling), dinosaurs and [[mammal]]s (erect), and [[rauisuchia]]ns (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 [[diapsid]]s, dinosaurs ancestrally had two pairs of [[Infratemporal fenestra]]e (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the [[antorbital fenestra|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">{{cite web |url=https://ucmp.berkeley.edu/diapsids/dinomm.html |last1=Smith |first1=Dave |display-authors=et al. |title=Dinosauria: Morphology |publisher=[[University of California Museum of Paleontology]] |location=Berkeley |access-date=October 16, 2019 |archive-date=November 12, 2020 |archive-url=https://web.archive.org/web/20201112035340/https://ucmp.berkeley.edu/diapsids/dinomm.html |url-status=live }}</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 [[Evolution of mammals#Erect limbs|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 [[Carrier's constraint|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 [[rauisuchia]]ns, 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 [[ilium (bone)|upper pelvic bone]] was rotated to form an overhanging shelf.<ref name=TKMB07/>