an old-style representation of a thecodont (Ornithosuchus)
The tendency nowdays is to see them as mostly quadrupedal rather than bipedal
model from the Elgin Museum
Archosaurian systematics is a constant state of flux, as various paleontological paradigms and sub-paradigms have came into and out of favour. Regarding the relationships of the basal archosaurs or Thecodontia, a number of different hypotheses have been offered over the last half century, any or none of which may be correct.
Until quite recently the Thecodontia (following the Linenan Evolutionary-Systematic approach) were considered a valid order divided into four suborders, the Proterosuchia, Phytosauria, the Aetosauria, and the Pseudosuchia. The Proterosuchia include various primitive ancestral types, the Phytosaurs are crocodile like forms, the Aetosaurs armoured creatures, remarkably like the armoured ornithischian dinosaurs, and the Pseudosuchia is a sort of catch-all or "waste-basket" category for everything that doesn't fit in any of the other suborders. This last group is also considered ancestral to dinosaurs, birds, and crocodiles.
Thecodontia
- presents the old four suborder division. Some errors on this page
- a few families presented under Pseudosuchia actually belong to the other
suborders!
Although very similiar anatomically and clearly related to each other by common ancestry and many shared ancestral traits (plesiomorphies), the thecodonts are nowadays no longer considered a single taxon, but rather are divided along cladistic (ancestor-descendent) lines. Cladistics however offers no more objectivity than Evolutionary Systematics, as indicated by the rapidly changing cladistic paradigms, any (or all, or none) of which may be correct
One cladistic hypothesis has it that archosaurs (including thecodontia) can be classified according to whether they had crocodile-type or bird-type ankles. The Crocodile-type ankle ones are called Crocodylotarsi. This group includes the unfortunately named Pseudosuchia ("false crocodiles") and the Crocodylia. The bird-ankled types are called Ornithosuchia or Dinosauromorpha, and include the former "pseudosuchian" Ornithosuchidae, the Dinosaurs, and the birds. The Pterosaurs may also be included here. The Proterosuchia are too primitive to belong to either the crocodile or bird ankled group. Here are two cladograms (reproduced in Lars Juul's article "The phylogeny of basal archosaurs", Palaeont. Afr., 1994,) presenting this hypothesis.
J. Gauthier, 1986, "Saurischian
Monophyly and the origin of birds,"
in Padian K,. Ed., "The Origin of Birds and the
Evolution of Flight", 1-55, 8, Memoirs, California Academy of
Sciences, San Francisco
M.J. Benton and J.M. Clark 1988
"Archosaur phylogeny and the relationships of the Crocodilia" in MJ Benton
(ed.), The Phylogeny and Classification of the Tetrapods 1:295-338.
Oxford, The Systematics Association
The hypothesis presented in the above two cladograms was replaced by an
alternative one when it was discovered (or decided) that the Ornithosuchus
ankle is actually more like the Pseudosuchian one. This was argued
in 1994 by Lars Juul, in The phylogeny of basal archosaurs. pp.
1-38. (see Lars Juul, 1994, "The phylogeny of basal archosaurs", Palaeont. Afr.,
1994). This very technical paper that presents a subdivision of the
early Archosauria following the cladistic method. This hypothesis
again divides the Archosauria into two groups on the basis of their ankle
structure: the Crurotarsi (all thecodontia apart from the Protosuchia)
and the Ornithodira (dinosaurs, birds, lagosuchus and pterosaurs).
The pseudosuchians are here presented as un unresolved trichotomy with
with one group having prestosuchids and aetosaurs, a second with phytosaurs
(Parasuchia) and a third, new, group - the
Dromaeosuchia - which includes the Ornithosuchidae
and the Paracrocodylomorpha (Crocodylomorpha plus Gracilisuchus and Poposaurids).
The Ornithodira again is made up of pterosaurs, Lagosuchids and Dinosaurs.
The paper is mentioned by Ralph Chapman at 
A
few more.... Professor Paul Eric Olsen has a simplified version
of teh cladogeram (and good introduction to the various reptilain types
referred therein) on his page Great
Triassic Assemblages Pt 1 - The Chinle and Newark. Here is the
actual cladogram from the article:
Archosauriformes
- has a cladogram and information on ankle structure and other features
used in basal archosaurian classification, also argues that Ornithosuchidae
may not belong in Crurotarsi - part of Jack Conrad's Vertebrate
Phylogeny site
Archosauromorphs,
Basal Archosaurs, Basal Crurotarsi - material by Toby White and yours truely - this page
features a cladogram and information on basal archosaur relationships,
also includes notes on different thecodontian taxa
The following table combines a number of these different interpretations:
| Order Thecodontia
a. Clade Archosauriformes Suborder Proterosuchia Family Proterosuchidae Family Erythosuchidae Family Euparkeriidae (or under Ornithosuchia)? b. Clade Archosauria (consisting of Curotarsi and Ornithodira) c1.
Clade Crurotarsi
c2.
Clade Ornithodira
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