Crurotarsi

Crurotarsans Fossil range: Triassic - Recent, 250–0 Ma PreЄ Є O S D C P T J K Pg N
Clockwise from top-left: Longosuchus meani (an aetosaur), Angistorhinus grandis, (a phytosaur), Saurosuchus galilei (a rauisuchian), Pedeticosaurus leviseuri (a sphenosuchian), Chenanisuchus lateroculi (a eusuchian), and Dakosaurus maximus (a thalattosuchian).
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Class:	Reptilia
Subclass:	Diapsida
Infraclass:	Archosauromorpha
(unranked):	Archosauria
(unranked):	Crurotarsi Sereno & Arcucci, 1990
Orders
†Aetosauria Crocodylomorpha †Phytosauria †Rauisuchia*

The Crurotarsi ("cross-ankles") are a group of archosaurs, whose name was erected as a node-based clade by Paul Sereno in 1991 to supplant the old term Pseudosuchia. Crurotarsi are by definition the sister group of the Avemetatarsalia (all forms closer to birds than crocodiles).

Description

Crurotarsi is one of the two primary daughter clades of the Archosauria. The skull is often massively built, especially in contrast to ornithodires; the snout is narrow and tends to be elongated, the neck is short and strong, and the limb posture ranges from a typical reptilian sprawl to an erect stance like that of dinosaurs or mammals (although crurotasans achieve this in a different way). The body is often protected by two or more rows of armored plates. Many crurotarsans reached lengths of three meters or more.

Evolution

Crurotarsans 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 crocodilians Sphenosuchia, and a few other assorted groups.

At the end Triassic extinction, all of the large crurotarsans died out. The cause of their extinctions is yet unknown, but it is believed that it may have been an asteroid impact, sudden climate change, or a planetary cataclysm. Furthermore, this allowed the dinosaurs to succeed them as the dominant terrestrial carnivores and herbivores. Only the Sphenosuchia and the Protosuchia (Crocodylomorpha) survived.

As the Mesozoic progressed, the Protosuchia gave rise to more typically crocodile-like forms. While dinosaurs were the dominant animals on land, the crocodiles flourished in rivers, swamps, and the oceans, with far greater diversity than they have today.

With the end Cretaceous extinction the dinosaurs became extinct, with the exception of the birds, while the crurotarsan crocodilians continued with little change.

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

Phylogeny

Cladogram after Parrish (1993), Nesbitt (2003 & 2005), and Nesbitt & Norell (2006).[1]

See also

• List of crurotarsans

References

• Benton, M. J. (2004, 3rd ed.). Vertebrate Paleontology. Blackwell Science. 
• Sereno, Paul (1991). "Basal archosaurs: phylogenetic relationships and functional implications". Journal of Vertebrate Paleontology (Suppl.) 11: pp. 1–51. 
• "Lucky Break allowed Dinosaurs to rule the Earth study". Yahoo. 2008. http://news.yahoo.com/s/nm/20080911/sc_nm/dinosaurs_dc. Retrieved 2008-09-13. 

External links

Wikispecies has information related to: Crurotarsi

• Palaeos
• taxonomic hierarchy according to Benton 2004
• Mikko's Phylogeny