Thecodont ('socket-toothed' reptile), is a catch-all ("paraphyletic") group, now considered an obsolete term, that was formerly used to describe a group of the earliest archosaurs that lived during the Permian and Triassic periods. See Archosaur for more information.
The 'Thecodonts' are not now considered monophyletic, that is, descended from a single ancestor. The group included ancestors of dinosaurs, (including birds), and ancestors of pterosaurs, and crocodilians, groups that diverged during the early Triassic.
The primitive dinosaurs of the Triassic were neither as abundant nor as varied as they would become in Jurassic and Cretaceous times.
The dinosaurs are a large group of reptiles belonging to the Archosauria ("ruling reptiles"), which also includes the pterosaurs, or "winged lizards"; crocodilians, the only surviving archosaurs; and the thecodonts, primitive "socket tooth" archosaurs, who were the ancestors of all other archosaurs.
The thecodonts were still very common, but died out by the end of the Triassic.
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