Archosauria

(vertebrate zoology) A subclass of reptiles composed of five orders: Thecodontia, Saurischia, Ornithschia, Pterosauria, and Crocodilia.

A subclass of reptiles more easily defined by listing its five component orders, four of them extinct, than by its anatomical characters. The earliest, most primitive archosaurs were the Thecodontia, from which arose independently the Saurischia (“lizard-hipped” dinosaurs), the Omithischia (“bird-hipped” dinosaurs), the Pterosauria (flying reptiles, including the pterodactyls), and the Crocodilia (including the living crocodiles, alligators, caimans, and gavials). See also Crocodylia; Ornithischia; Pterosauria; Saurischia.

Many Archosauria are certainly bipeds. They have two pairs of openings (fenestrae) in the temporal region of the skull (diapsid condition) and never lose the bony arcades around those openings. Typically there are also one or more antorbital openings in front of the orbit, a character which is virtually confined to this subclass (see illustration).

archosaur (order Thecodontia), from the Early Triassic of South Africa. (After Broili and Schroeder)">
Skull of Chasmatosaurus, a primitive archosaur (order Thecodontia), from the Early Triassic of South Africa. (After Broili and Schroeder)

The earliest archosaurs are Lower Triassic, with a single species in the uppermost Permian. The Archosauria were undoubtedly a highly successful group, with locomotor systems, sense organs, and probably physiological adaptations far superior to those of any otherreptiles. See also Reptilia.