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link to palaeos com Kingdoms of Life > Wikipedia link Animalia > link to palaeos com Vertebrata > link to palaeos com Tetrapoda > Wikipedia link Reptilia/link to palaeos com Sauropsida > link to palaeos com Archosauria > Thecodontia

Author's note: these pages were written some years ago. I am not planning to update them. For a more current coverage, see the link to palaeos com Palaeos website (to which many links on these pages point to anyway. More info here

Thecodontian/Basal Archosaurian Systematics - changing perspectives

Ornithosuchus - an old-style reconstruction
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.

Four Suborders

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 link to palaeos com crocodiles.

Illinois State Academy of ScienceThe Kingdoms ProjectThecodontia - 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

Crocodile and Bird-type ankles

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.

Gauthier cladogram

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

Benton and Clark cladogram
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


Crurotarsi and Ornithodira

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 external link 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  external link Great Triassic Assemblages Pt 1 - The Chinle and Newark.  Here is the actual cladogram from the article:

Archosauria cladogram
Lars Juul's Archosaur Cladogram  - from "The phylogeny of basal archosaurs", Palaeont. Afr., 1994, p.22

external link 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

link to palaeos com 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
                   Family Proterochampsidae?
          d1. Clade Parasuchia
    Suborder Parasuchia (Phytosauria)
          d2. Clade "Rauisuchia"
   Suborder Rauisuchia (Pseudosuchia in part)
                   Family Ctenosauriscidae?
                   Family Ornithosuchidae (or under Ornithosuchia)?
                Family Prestosuchidae
                   Family Rauischidae
   Suborder Aetosauria (Pseudosuchia in part)
                   Family Stagonolepidae
   Suborder "Dromaeosuchia"
         d3. Clade Dromaeosuchia
                   Family Erpetosuchidae?
             e1. Clade Paracrocodylomorpha
                   Family Gracilisuchidae
                   Family Postosuchidae (Poposauridae)
(note: some forms previously included among the Pseudosuchia have been transferred to the suborder Sphenosuchia of the Crocodylia)
         Order Crocodylia

     c2. Clade Ornithodira
    Suborder Ornithosuchia (polyphyletic (invalid) group?)
                  Family Euparkeriidae (or under Proterosuchia)?
                Family Ornithosuchidae (or under Dromaeosuchia)?
                 Family Lagosuchidae (or under Dinosauria or seperate suborder/order)?
    Order Pterosauria
    Superorder Dinosauria

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page by M.Alan Kazlev
page uploaded 19 December 1998. Reposted and last modified 30 August 2005, links updated 16 January 2010