Archosaurs (Greek for "ruling reptiles") are a group of diapsid reptiles that first appeared during the late Permian (roughly 250 million years ago). They were predatory semi_aquatic animals, with a sprawling elbows_out stance and long snouts. Their narrow serrated meat_tearing teeth were set in sockets in their jaws, a feature that inspired the traditional name, "thecodonts" that used to be given to early Triassic archosaurs. In some way they were adapted to the strong monsoon climates that were the natural result of the single world-continent, Pangaea. Some of them survived the catastrophe at the end of the Permian, perhaps because they were opportunistic scavengers, perhaps because they could retreat into water to find respite from an overheated climate. Any such scenarios are hypothetical; what is clearer is that within a few millions years at the opening of the Triassic, they had evolved rapidly to fill empty ecological niches in the devastated global system.

There are two primary groups of archosaurs — the ornithosuchians, which include the ancestors of dinosaurs and pterosaurs; and the pseudosuchians, which include the crocodylians forebears. They are set apart by having socketed teeth and four-chambered hearts, among other characteristics. The last few years Archosauria has been more commonly divided into the Crurotarsi and the Ornithodira, the last group containing among related forms the Dinosauria and the Pterosauria, but not the Ornithosuchids.

Most archosaur groups became extinct 195 million years ago, during the Triassic-Jurassic extinction event; most of their surviving descendent then perished 65 million years ago, during the Cretaceous_Tertiary extinction event. The only groups of archosaurs with living members are the birds, which are descended from the dinosaurs, and crocodylians, which include all modern crocodiles, alligators, and gharials.

Birds are traditionally treated as a separate class, Aves, while the rest of the archosaurs are treated as a subclass, Archosauria, within the class Reptilia. Biologists who prefer monophyletic groups, however, may treat them all in a separate class Archosauria.

External link

• Paleos.com (http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.400.html#Archosauriformes) Archosauromorpha, with further links