archosaur, (subclass Archosauria), any of various reptiles, including allcrocodiles andbirds and all descendants of their most recent common ancestor. Archosaurs (“ruling reptiles”) are members of a subclass that also includes thedinosaurs, thepterosaurs (flying reptiles), and several groups of extinct forms, mostly from theTriassic Period (251 million to 200 million years ago).

The true archosaurs are divided into two branches. ThePseudosuchia branch includes crocodiles and all other archosaurs more closely related to crocodiles than to birds. Pseudosuchianscomprise mostly extinct Triassic groups such asphytosaurs, aetosaurs, prestosuchids, rauisuchids, and poposaurs. All were carnivorous except the armoured, herbivorous aetosaurs. The second archosaurian branch, theOrnithosuchia, includes birds and all archosaurs more closely related to birds than to crocodiles. In addition to the dinosaurs (the group from which birds evolved and to which they formally belong), ornithosuchians include pterosaurs and some extinct Triassic forms such as lagosuchids and lagerpetontids.

The first known archosaurs appeared in the Middle Triassic Period (about 246 million to 229 million years ago). They evolved from an earlier group of diapsid reptiles, diapsids having two openings in the skull behind the eye. These, along with protorosaurs, rhynchosaurs, trilophosaurids, proterosuchians, erythrosuchids, and proterochampsids, belong to a larger assemblage called the Archosauromorpha. Most archosaurs had long hind legs and short forelimbs. All archosaurs had a large opening of uncertain function on the snout in front of the eye (antorbital fenestra) and another at the junction of three bones in thelower jaw (mandibular fenestra). Unlike other living reptiles, whose teeth are set in a shallow groove, the teeth of archosaurs are set in sockets.

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Although most archosaurs were egg layers, there is evidence that some species gavebirth to live young. The oldest known example of such a live-bearing form isDinocephalosaurus, an archosauromorph—that is, a form more closely related to archosaurs than lepidosaurs (thelineage containing modern lizards and snakes, their direct ancestors, and their close relatives)—which lived about 245 million years ago.

The two branches of archosaurs developed different patterns of locomotion. The pseudosuchians have a generally crocodile-like ankle that flexes in the middle so the foot can twist against the ankle as it walks sprawled out. However, thisankle joint can also stiffen so that theanimal moves in a “high walk,” as seen in crocodiles. The ornithosuchians, on the other hand, have an ankle that swings in only one plane (fore and aft), so they areobliged to walk upright and put one foot in front of the other as birds do. These differences seem to have characterized the archosaurian branches from the earliest times in their history. In correlation with this difference, nearly all pseudosuchians are typically quadrupedal, walking on four legs, whereas nearly all ornithosuchians are typically bipedal, walking on two.

The Triassic saw the first broad diversification of archosaurs, and, although most groups became extinct by the end of that period, dinosaurs, crocodiles, and pterosaurs survived and proliferated during the Jurassic andCretaceous periods. Birds evolved in the Late Jurassic, and, although the other dinosaurs and the pterosaurs became extinct by the end of the Cretaceous, the crocodiles and birds still remain.