Thecodont, name formerly used for various groups of advanced reptiles that lived during the Triassic period of Earth history. These prehistoric reptiles are now classified as archosaurs, the group of vertebrate animals that also includes the extinct dinosaurs and pterosaurs (flying reptiles), and the living crocodiles and birds. The name is derived from Greek words meaning “socket-toothed,” alluding to the fact that thecodont teeth were implanted in sockets in the jawbones.

New fossil discoveries have shown that the old concept of “thecodonts” wrongly grouped together animals that looked superficially similar but were not closely related. Different types of early archosaurs could walk erect on their two hind legs and had other dinosaur-like features. However, some archosaurs that walked on two legs were closely related to crocodiles, and were not the ancestors or close relatives of dinosaurs.

See also Evolution.