Crurotarsi ("cross-ankles") is a node-based taxon created by Paul Sereno in 1991 to supplant the old term Pseudosuchia . As the basal phylogeny of this clade is not really understood, a stem-based definition is now preferred: all forms closer to Crocodiles than to Birds. As such, the Crurotarsi are the opposite branch to the Ornithodira (all forms closer to Birds than Crocodiles). In terms of the old Linnean taxonomy they correspond more or less to the Orders Thecodonta (minus the Proterosuchia ) and Crocodylia

The Crurotarsi are one of the two primary daughter clades of the Archosauria. The skull is often massively built, especially in contrast to ornithodires; the snout narrow and sometimes tending to be elongate, the neck is short and strong, and the limb posture ranging from typically reptilian sprawing to dinosaur or mammal-like erect (although this is achieved in a different way to dinosaurs and mammals). The body is often protected by two or more rows of armoured plates. Many forms reached large size; 3 meters or more in length.

Crurotarsans seem to have appeared during the late Olenekian (early Triassic); by the Ladinian (late Middle Triassic) they dominated the terrestrial carnivore niches. Their heyday was the Late Triassic, during which time their ranks included erect-limbed rauisuchians, the crocodile-like phytosaurs, herbivorous armoured aetosaurs , the large predatory poposaurs , the small agile protocrocodilian Sphenosuchia , and a few other assorted groups.

The end Triassic extinction, all of the large crurotarsans died out, allowing the dinosaurs to succeed them as rulers of the land. Only the little Sphenosuchia and the Protosuchia (Crocodylia) survived.

As the Mesozoic proceeded, the Protosuchia gave rise to more typically crocodile like forms (the Crocodylia proper), and, while Dinosaurs were the dominant animals on land, the crocodiles flourished in rivers, swamps, and the ocean; with far greater diversity than they have today.

With the end Cretaceous extinction the tables were turned - the ornithodiran dinosaurs died out (although their descendents the birds survived) while the curotarsan crocodylians continued with little change.

Today, the crocodiles, alligators, and gavials continue as the surviving representatives of this ancient and successful lineage.

External links

• EvoWiki

• Palaeos

• taxonomic hierarchy according to Benton 2004

• Mikko's Phylogeny

References

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd

• Sereno, P. (1991), Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology (Suppl.) 11: 1-51.