Thecodontia

The group includes the ancestors of dinosaurs, pterosaurs, and crocodilians, as well as a number of extinct forms that did not give rise to any descendants.

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, a wastebasket taxon intended to be paraphyletic to all later archosaurs (see e.g., Alfred Sherwood Romer's Vertebrate Paleontology and Edwin H. Colbert's Evolution of the Vertebrates).

Robert Carroll, in his book Vertebrate Paleontology and Evolution (1988), replaces Pseudosuchia with Rauisuchia, Ornithosuchia (containing Ornithosuchidae and non-dinosaur, non-pterosaur Ornithodira), and the traditional category incertae sedis (of uncertain placement), while retaining the other three suborders.

In his 1982 thesis on amniote phylogeny, Brian Gardiner attempted to define Thecodontia within a cladistic framework, containing crocodilians and haemothermata (mammals and birds), thus giving the old name to a new concept.

All more recent cladistic studies (e.g., Jacques Gauthier 1986) have confirmed that Thecodontia as construed by Gardiner is a polyphyletic taxon, the members of which are not united by any shared derived characteristics.