Editing Archosaur (section)

==Main forms==
[[File:Pancrocodylia diversity.jpg|thumb|left|200px|Examples of pseudosuchians. Clockwise from top-left: ''[[Longosuchus|Longosuchus meadei]]'' (an [[aetosaur]]), ''[[Gavialis gangeticus]]'' (a [[crocodilian]]), ''[[Saurosuchus|Saurosuchus galilei]]'' (a [[loricata]]n), ''[[Pedeticosaurus|Pedeticosaurus leviseuri]]'' (a [[sphenosuchia]]n), ''[[Chenanisuchus|Chenanisuchus lateroculi]]'' (a [[Dyrosauridae|dyrosaurid]]), and ''[[Dakosaurus|Dakosaurus maximus]]'' (a [[thalattosuchia]]n).]]
[[File:Panaves diversity.jpg|thumb|right|200px|Examples of avemetatarsalians. Clockwise from top-left: ''[[Tupuxuara|Tupuxuara leonardi]]'' (a [[pterosaur]]), ''[[Alamosaurus|Alamosaurus sanjuanensis]]'', (a [[Sauropoda|sauropod]]), ''[[Tsintaosaurus|Tsintaosaurus spinorhinus]]'' (an [[Ornithopoda|ornithopod]]), ''[[Daspletosaurus|Daspletosaurus torosus]]'' (a [[Tyrannosauridae|tyrannosaurid]]), ''[[Pentaceratops|Pentaceratops sternbergii]]'' (a [[ceratopsia]]n), and ''[[Common crane|Grus grus]]'' (a [[neornithes|neornithian]]).]]

Since the 1970s, scientists have classified archosaurs mainly on the basis of their ankles.<ref>[http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html Archosauromorpha: Archosauria - Palaeos] {{webarchive|url=https://web.archive.org/web/20050405074210/http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html |date=2005-04-05 }}</ref> The earliest archosaurs had "primitive mesotarsal" ankles: the [[Talus bone|astragalus]] and [[Calcaneus|calcaneum]] were fixed to the [[tibia]] and [[fibula]] by [[Suture (anatomical)|suture]]s and the joint bent about the contact between these bones and the foot.

The [[Pseudosuchia]] appeared early in the [[Triassic]]. In their ankles, the astragalus was joined to the tibia by a [[suture (anatomical)|suture]] and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some later crurotarsans developed fully erect limbs. Modern crocodilians are crurotarsans that can employ a diverse range of gaits depending on speed.<ref>{{cite journal |last1=Hutchinson |first1=John R. |last2=Felkler |first2=Dean |last3=Houston |first3=Kati |last4=Chang |first4=Yu-Mei |last5=Brueggen |first5=John |last6=Kledzik |first6=David |last7=Vliet |first7=Kent A. |date=2019-12-17 |title=Divergent evolution of terrestrial locomotor abilities in extant Crocodylia |journal=Scientific Reports |language=en |volume=9 |issue=1 |pages=19302 |doi=10.1038/s41598-019-55768-6 |pmid=31848420 |pmc=6917812 |bibcode=2019NatSR...919302H |issn=2045-2322}}</ref>

''[[Euparkeria]]'' and the [[Ornithosuchidae]] had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus.

The earliest fossils of [[Avemetatarsalia]] ("bird ankles") appear in the [[Anisian]] age of the [[Middle Triassic]]. Most Ornithodirans had "advanced mesotarsal" ankles. This form of ankle incorporated a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. The earliest avemetatarsalians, such as Teleocrater and Asilisaurus, retained "primitive mesotarsal" ankles. The ornithodirans differed from other archosaurs in other ways: they were lightly built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornithodirans were completely [[bipedalism|bipedal]]. The archosaurian fourth trochanter on the femur may have made it easier for ornithodirans to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic, the ornithodirans diversified to produce [[dinosaur]]s and [[pterosaur]]s.