Editing Dinosaur (section)

===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/>