Pseudosuchia

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Pseudosuchia ("false crocodiles") is the name originally given to a group of prehistoric reptiles from the Triassic period. The name has been variously interpreted, and it is still sometimes, if infrequently, used in scientific literature today.

[edit] Taxonomic History

The name Pseudosuchia was coined by Karl Alfred von Zittel in 1887-1890 to include three taxa (two aetosaurs and Dyoplax) that were superficially crocodilian-like, but were not actually crocodilian. Hence the name "false crocodiles".

In mid-twentieth century textbooks like Romer's Vertebrate Paleontology and Colbert's Evolution of the Vertebrates the Pseudosuchia constitute one of the suborders of the Thecodontia. Zittel's aetosaurs were now placed in their own suborder. For typical Pseudosuchians, Colbert uses the example of small lightly-built archosaurs such as Ornithosuchus and Hesperosuchus, both of which were at the time reconstructed as bipeds like miniature (length about 1 meter) Theropods. These little animals were assumed to be the ancestors of all later archosaurs. The name however also became a sort of wastebasket taxon for all thecodonts that didn't fit in the other three suborders. Even Sharovipteryx and Longisquama have been regarded as pseudosuchians.

Under the cladistic system, Pseudosuchia was recognised as a paraphyletic assemblage, and hence invalid. Some writers however, such as Gauthier and Padian (1985) and Gauthier (1986) have tried to establish this name in a cladistic context, using it as a monophyletic stem-based taxon for crocodile-ancestors and their descendants (including crocodiles, which makes the original name somewhat ironic). These definitions include "Crocodiles and all archosaurs closer to crocodiles than to birds" (Gauthier and Padian), "Extant crocodiles and all extinct archosaurs that are closer to crocodiles than they are to birds" (Gauthier 1986), and more recently "the most inclusive clade within Archosauria that includes Crocodylia but not Aves" (Senter 2005).

A different definition was suggested by Benton and Clark, 1988: the node-based taxon including the Rauisuchidae and the aetosaurs. However, relations between those two groups and other archosaurs are controversial, and so this clade may be invalid (or include living crocodiles).

Paul Sereno rejects the term Pseudosuchia, and it is not often applied; his Crurotarsi being more often used instead (another partial synonym is Benton and Clark's Crocodylotarsi).

[edit] Bibliography

  • Benton, MJ & JM 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
  • Gauthier, J., 1986. Saurischian monophyly and the origin of birds. In: K. Padian, ed. The Origin of Birds and the Evolution of Flight. Memoirs California Academy of Sciences 8. pp. 1–55
  • Gauthier, J. and K. Padian, 1985. Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight. Pp. 185-197 In: M.K. Hecht, J.H. Ostrom, G. Viohl and P. Wellnhofer, eds. The Beginnings of Birds: proceedings of the international Archaeopteryx conference; 1984; Eichstätt, Germany. Eichstätt: Freunde des Jura-Museums. pp. 185–197.
  • Senter, P. 2005. Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. PaleoBios, v. 25, n. 2, p. 1-7.
  • Sereno, P. C. 2005. Stem Archosauria—TaxonSearch [version 1.0, 2005 November 7]

[edit] External links

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