Crurotarsi

Crurotarsans Temporal range: Early Triassic–Present,
Life restoration of Protome batalaria
Life restoration of Ornithosuchus woodwardi
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Clade:	Eucrocopoda
Clade:	Crurotarsi Sereno & Arcucci, 1990
Subgroups
†Phytosauria Archosauria Avemetatarsalia Pseudosuchia

Crurotarsi is a group of archosauriform reptiles that includes the archosaurs (represented today by birds and crocodilians) and the extinct, crocodile-like phytosaurs. The name is derived from the Latin word crus and the Greek word tarsos; it refers to the specialized articulation between crus and tarsus—specifically between fibula and calcaneum—present in the skeletons of suchians and phytosaurs, with a hemicylindrical condyle on the calcaneum articulating against fibula.

Taxonomic history

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, but with a different definition. Crurotarsi includes, by most published definitions, all descendants of the common ancestor of modern crocodiles, ornithosuchids, aetosaurs, and phytosaurs; Nesbitt (2011) provided a shorter definition, defining Crurotarsi as "the least inclusive clade containing Rutiodon carolinensis Emmons, 1856, and Crocodylus niloticus Laurenti, 1768". According to two studies published in 2011 by Nesbitt and coworkers, using either of these definitions leads to the inclusion of all other true archosaurs in Crurotarsi, due to the possibly basal phylogenetic position of the phytosaurs. This means that grouping the phytosaurs and crocodilians into a clade while excluding the avemetatarsalians (pterosaurs, dinosaurs, and birds) would result in a paraphyletic grouping. A more definitive group is Pseudosuchia, which is defined as all archosaurs closer to crocodiles than to birds (matching the traditional content of Crurotarsi).

Phylogeny

Paul Sereno and A. B. Arcucci named Crurotarsi in 1990, defining it as "Parasuchia [phytosaurs], Ornithosuchidae, Prestosuchus, Suchia, and all descendants of their common ancestor". The groups in this definition were considered crocodile-line archosaurs, as opposed to the bird-line archosaurs. Ornithosuchids were once considered bird-line archosaurs (as implied by their name, which means "bird crocodiles" in Greek), but were later recognized as crocodile-line archosaurs. This reclassification may have inspired Sereno's Crurotarsi, a node-based clade defined by the inclusion of ornithosuchids and other early archosaurs.

Two names were proposed for crocodile-line archosaurs before Crurotarsi was erected. The first, Pseudosuchia, was established as a stem-based clade in 1985. It includes crocodiles and all archosaurs more closely related to crocodiles than to birds. The second, Crocodylotarsi, was named in 1988, possibly as a replacement for Pseudosuchia. The name Pseudosuchia, meaning "false crocodiles", has been used for over a century, and traditionally included aetosaurs. As a clade, Pseudosuchia includes the group Eusuchia, or "true crocodiles". Crocodylotarsi may have been named to remove confusion, but as a stem-based clade it is synonymous with Pseudosuchia. Because Pseudosuchia was named first, it has precedence. Crurotarsi traditionally contains the same archosaurs as Pseudosuchia, but as a node-based clade it is not synonymous.

In 2011, Sterling J. Nesbitt found phytosaurs to be the sister taxon of Archosauria, and therefore not crocodile-line archosaurs. Because phytosaurs are included in the definition of Crurotarsi, this change in their phylogenetic placement expanded the scope of Crurotarsi, which therefore now includes phytosaurs, crocodiles, pterosaurs and dinosaurs. However, Pseudosuchia still contains only crocodile-line archosaurs.

Below is a cladogram modified from Nesbitt (2011) showing the new changes:

Archosauriformes	†Proterosuchidae †Erythrosuchidae †Vancleavea †Proterochampsia †Euparkeria  Crurotarsi  †Phytosauria  Archosauria  Avemetatarsalia (bird-line archosaurs)  Pseudosuchia (crocodile-line archosaurs)  †Ornithosuchidae Suchia

Below is a cladogram after Nesbitt & Norell (2006) and Nesbitt (2007) with Crurotarsi in its traditional sense encompassing just crocodile-line archosaurs:

Archosauriformes	†Euparkeria †Proterochampsidae  Archosauria  to Avemetatarsalia Crurotarsi  †Phytosauria  Suchia  †Aetosauria Crocodylomorpha †Ornithosuchidae  Rauisuchia  †Rauisuchidae †Prestosuchidae  "Group X"  †Arizonasaurus †Lotosaurus  "Group Y" or Shuvosaurinae  †Sillosuchus †Shuvosaurus †Effigia

Cladogram after Brusatte, Benton, Desojo and Langer (2010)

Archosauriformes

†Erythrosuchus †Euparkeria †Proterochampsidae  Archosauria  to Avemetatarsalia Crurotarsi  †Phytosauria  Suchia  †Aetosauria  Paracrocodylomorpha  †Gracilisuchus  Bathyotica  †Erpetosuchus Crocodylomorpha †Revueltosaurus †Ornithosuchidae  Rauisuchia   †Rauisuchoidea  Arganasuchus Fasolasuchus Stagonosuchus Ticinosuchus  Prestosuchidae  Saurosuchus Batrachotomus Prestosuchus  Rauisuchidae  Tikisuchus Rauisuchus Postosuchus Teratosaurus  †Poposauroidea  Yarasuchus Qianosuchus Arizonasaurus Bromsgroveia Lotosaurus Poposaurus Sillosuchus  Shuvosauridae  Shuvosaurus Effigia

References

• Benton, M. J. (2004). Vertebrate Paleontology (3rd ed.). Blackwell Science.
• Dunham, W. (2008-09-12). "Lucky break allowed dinosaurs to rule Earth". Reuters. Retrieved 2012-01-14.
• Brusatte, S. L.; Benton, M. J.; Ruta, M.; Lloyd, G. T. (2008-09-12). "Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs" (PDF). Science. 321 (5895): 1485–1488. doi:10.1126/science.1161833. PMID 18787166. Retrieved 2012-01-14.

External links

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