Thecodonta

Thecodont ("socket-toothed" reptile), now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Late Permian and flourished until the end of the Triassic Period. The group includes the ancestors of dinosaurs (including birds), and ancestors of pterosaurs, and crocodilians, as well as a number of extinct forms that did not give rise to any descendants.

Definition
Thecodonts are defined by certain shared primitive features, such as the suborbital fenestra (an opening on each side of the skull between the eye sockets and the nostrils) and teeth in sockets. The name thecodont is Greek for "socket-tooth," referring to the fact that thecodont teeth were set in sockets in the jawbones; an archosaurian characteristic that was inherited by the dinosaurs.

They constitute an evolutionary grade of animals, a "wastebasket taxon" for any archosaur that wasn't a crocodilian, a pterosaur, or a dinosaur (i.e. any basal archosaur). Because the cladistic paradigm only recognises monophyletic taxa as natural groups, and because Thecodonts are a paraphyletic group (i.e. they include among their descendants animals that are not thecodonts), the term is no longer used by most paleontologists, although it can still be found in older (and even fairly recent) books.

Taxonomic History
Traditionally, the order Thecodontia Owen, 1859 was divided into four suborders, the Proterosuchia (early primitive forms, another paraphyletic assemblage), Phytosauria (large crocodile-like semi-aquatic animals), the Aetosauria (armoured herbivores), and the Pseudosuchia (see e.g. Alfred Sherwood Romer's Vertebrate Paleontology and Edwin H. Colbert's Evolution of the Vertebrates). Of these, only Phytosaurs and Aetosaurs constitute monophyletic groups, and the term Pseudosuchia was simply a catch-all term for any species that didn't fit in one of the other three sub-orders.

Robert Carroll in his book Vertebrate Paleontology and Evolution (1988) replaces Pseudosuchia with Rauisuchia, Ornithosuchia, and the traditional category incertae sedis (of uncertain placement), while retaining the other three suborders. This is the last major textbook that still recognises the taxon Thecodontia, as it uses a traditional Linnaean based taxonomy.

Brian Gardiner (1982) attempted to define Thecodontia within a cladistic framework, thus giving the old name to a new concept. All recent cladistic studies (e.g. Jacques Gauthier 1986) have confirmed that the traditional Thecodontia is indeed a paraphyletic taxon, the members of which are not united by any shared derived characteristics. As the association of the name with the outdated concept proved to be very strong, it is now considered a historical term only, and its current usage has been abandoned.

All current vertebrate paleontology textbooks, (e.g. Michael Benton's Vertebrate Palaeontology (first ed. 1990, 2nd ed. 1997), follow the cladistic approach, and so the name Archosauria is used instead. This includes both the Thecodonts and all their descendants.