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Thecodontia is a paraphyletic assemblage of basal archosaurs. The group has historically been a refugium for any non-crocodilian, non-pterosaurian, non-dinosaurian archosaur (Benton 1990), and classically defined (see Romer 1933, 1945, 1956, 1966, Carroll 1988) includes members of two distinctly different archosaurian clades, Ornithosuchia and Crurotarsi. "Thecodontia" lacks any synapomorphies with which to underwrite holophyly thereof, and as such, cannot be defined in phylogenetic terms.

The principal difficulty with "Thecodontia" is that it is diagnosed by the same characters which define Archosauria (non sensu Gauthier) and is thus a redundant term, as noted by Gauthier (1986). Consequently, a "thecodont origin" of birds is simply to say that birds are derived from somewhere within Archosauria.

It is worth noting, however, that the phylogeny of basal archosaurs and archosauromorphs remains extremely contentious and there has been an unfortunate tendency to neglect these forms classically lumped in "Thecodontia." They represent the wellsprings of the major archosaur adaptive radiations in the Triassic and require further study.

Thecodonts evolved from Eosuchians, descendants of early Archosaurs.

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