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Cladus: Eukaryota
Supergroup: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Superclassis: Tetrapoda
Classis: Reptilia
Subclassis: Diapsida
Infraclassis: Archosauromorpha
Divisio: Archosauria
Subsectio: Crurotarsi
Superordo: Crocodylomorpha - incertae sedis

Name

Crurotarsi (Sereno & Arcucci, 1990)

The Crurotarsi ("cross-ankles") are a group of archosaurs, represented today by the crocodiles, alligators and gavials and including many extinct forms. The name Crurotarsi was erected as a node-based clade by Paul Sereno and A.B. Arcucci in 1990 to supplant the old term Pseudosuchia.[1] Crurotarsi are by definition the sister group of the Avemetatarsalia (all forms closer to birds than crocodiles).


Description
Further information: Crurotarsal

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 crurotarsans 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. A study published in "Proceedings of the National Academy of Sciences of the United States of America" in 2010 postulates that there is significant evidence that volcanic eruptions changed the climate, causing a mass extinction that wiped out the dinosaurs' main competitors. 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]
Cladogram after Nesbitt & Norell (2006) [2] and Nesbitt (2007).[3]
Cladogram after Brusatte, Benton, Desojo and Langer (2010) [4]

References

1. ^ Sereno, P.C. and Arcucci, A.B. (1990). "The monophyly of crurotarsal archosaurs and the origin of bird and crocodile ankle joints." Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, 180: 21-52.
2. ^ Nesbitt SJ, Norell MA. 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society of London B 273: 1045–1048.
3. ^ Nesbitt S. 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History 302: 84 pp.
4. ^ Stephen L. Brusatte; Michael J. Benton; Julia B. Desojo; Max C. Langer. 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida). Journal of Systematic Palaeontology, 8: 1, 3 — 47pp. DOI: 10.1080/14772010903537732

* 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. [dead link]
* Whiteside, Jessica H.; Paul E. Olsen, Timothy Eglinton, Michael E. Brookfield, and Raymond N. Sambrotto (March 22, 2010). "Compound-specific carbon isotopes from Earth’s largest flood basalt eruptions directly linked to the end-Triassic mass extinction". PNAS 107 (15): 6721–5. doi:10.1073/pnas.1001706107. PMID 20308590. PMC 2872409. http://www.pnas.org/content/early/2010/03/15/1001706107.

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