ARCHOSAUR

any of the various advanced reptiles of the subclass Archosauria (Ruling Reptiles), including all thecodonts, pterosaurs (flying reptiles), dinosaurs, bird ancestors, and crocodilians, the only extant order. Early archosaurs appeared just prior to the beginning of the Triassic Period (245 to 208 million years ago). They probably derived from an earlier diapsid group and are associated with protorosaurs, rhynchosaurs, and trilophosaurids in a larger assemblage, the Archosauromorpha. All members of this assemblage were distinguished by an ankle specialization that facilitated upright posture. Most archosaurs had long hindlegs and short forelimbs. Nearly all were characterized by two pairs of openings in the skull above the eye (temporal fenestrae), to which jaw muscles were attached, and a large opening of uncertain function on the snout in front of the eye (antorbital fenestra). Unlike earlier reptiles, whose teeth were set in a shallow groove, the teeth of archosaurs were set in sockets. Archosaurus, the earliest archosaur known from the fossil record, dates from the Late Permian Period (258 to 245 million years ago) and was found in Russia.