Pseudosuchia

Pseudosuchia Temporal range: Triassic - Recent, 250–0 Ma PreЄ Є O S D C P T J K Pg N
Examples of extinct pseudosuchians. Clockwise from top-left: Longosuchus meani (an aetosaur), Angistorhinus grandis, (a phytosaur), Saurosuchus galilei (a rauisuchian), Pedeticosaurus leviseuri (a sphenosuchian), Chenanisuchus lateroculi (a dyrosaurid), and Dakosaurus maximus (a thalattosuchian).
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Node:	Crurotarsi
Branch:	Pseudosuchia Zittel, 1887
Subgroups
†Ornithosuchidae Suchia
Synonyms
Crocodylotarsi Benton & Clark, 1988

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. A more commonly used name, Crurotarsi, is often substituted for Pseudosuchia, as it generally includes the same taxa. Unlike Crurotarsi though, Pseudosuchia as used in recent literature is a stem-based taxon that includes crocodile-line archosaurs, or all archosaurs (including crocodilians) that are more closely related to crocodilians than birds.

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, Gauthier and Padian (1985) and Gauthier (1986) became the first to establish this name in a phylogenetic context, using it as a monophyletic stem-based taxon for crocodile-ancestors and their descendants. This made the name Pseudosuchia somewhat ironic, because true crocodiles are now included. Phylogenetic definitions of Pseudosuchia 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). As a stem-based taxon, Pseudosuchia is the sister taxon of another stem group, the Avemetatarsalia. Avemetatarsalians are bird-line archosaurs, including pterosaurs, non-avian dinosaurs, and all modern birds.

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 by him; his Crurotarsi is more often used instead (another partial synonym is Benton and Clark's Crocodylotarsi). Crurotarsi, however, is defined as a node-based taxon, relying on the inclusion of groups such as Phytosauria, Aetosauria, and Crocodylomorpha. Pseudosuchia and Crurotarsi have been considered partial synonyms because the latter clade encompasses all crocodile-line archosaurs in most phylogenetic analyses.^[1] However, some analyses such as Nesbitt (2011) place one crurotarsan group, Phytosauria, outside Pseudosuchia. Since the definition of Crurotarsi relies on phytosaurs, their placement outside Pseudosuchia (and thus Archosauria) means that the clade Crurotarsi includes both pseudosuchians and avemetatarsalians.^[2]

References

1. ^ Brochu, C.A. (1997). "Synonymy, redundancy, and the name of the crocodile stem-group". Journal of Vertebrate Paleontology 17 (2): 448–449. 
2. ^ Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades". Bulletin of the American Museum of Natural History 352: 1–292. doi:10.1206/352.1. http://digitallibrary.amnh.org/dspace/bitstream/2246/6112/1/B352.pdf. 

• 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, 7 November 2005]

External links

• Taxon Search - Pseudosuchia
• Re: Even more last papers for 2005 and follow up posts, on the Dinosaur Mailing List archives, for comments critical of applying "Pseudosuchia" in a cladistic context.