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Crurotarsans
Temporal range: Early Triassic - Holocene, 250–0 Ma
Scientific classification
Unrecognized taxon (fix): Crurotarsi
Subgroups

Crurotarsi is a group of archosauriformes that includes the archosaurs (represented today by crocodilians and birds) and the extinct, crocodile-like phytosaurs.[1] 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.[2][3]

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.[2] Crurotarsi include, 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".[1] According to two studies published in 2011, using one of these definitions leads to inclusion of all other true archosaurs in Crurotarsi, due to the possibly very primitive position of the phytosaurs, which means that phytosaurs and crocodilians do not form a clade that wouldn't also include avemetatarsalians (pterosaurs, dinosaurs, and birds). A more restrictive group defined as all archosaurs closer to crocodiles than to birds (matching the traditional content of Crurotarsi) is the Pseudosuchia.[1][4]

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."[2] 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.[5] 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.[6] 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.[7]

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:[1]

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:[8][9]

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) [10]

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

  1. ^ a b c d Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades" (PDF). Bulletin of the American Museum of Natural History. 352: 1–292. doi:10.1206/352.1. 
  2. ^ a b c Sereno, P.C.; 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. 
  3. ^ Sereno, Paul (1991). "Basal archosaurs: phylogenetic relationships and functional implications". Journal of Vertebrate Paleontology. (Suppl.) 11: pp. 1–51. 
  4. ^ Gauthier, J. A.; Nesbitt, S. J.; Schachner, E. R.; Bever, G. S.; Joyce, W. G. (2011). "The bipedal stem-crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion". Bulletin of the Peabody Museum of Natural History. 52: 107–126. doi:10.3374/014.052.0102. 
  5. ^ Gauthier, J.A.; Padian, K. (1985). "Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight". In Hecht, M.K.; Ostrom, J.H.; Viohl, G.; and Wellnhofer, P. (eds.). The Beginnings of Birds. Eichstatt: Freunde des Jura-Museums. pp. 185–197. 
  6. ^ Benton, M.J.; Clark, J.M. (1988). "Archosaur phylogeny and the relationships of the Crocodylia". In Benton, M.J. (ed.). Phylogeny and Classification of the Tetrapods. 1. Oxford: Clarendon Press. pp. 295–338. 
  7. ^ Brochu, C.A. (1997). "Synonymy, redundancy, and the name of the crocodlle stem-group". Journal of Vertebrate Paleontology. 17 (2): 448–449. doi:10.1080/02724634.1997.10010992. 
  8. ^ 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 (1590): 1045–1048. doi:10.1098/rspb.2005.3426. 
  9. ^ Nesbitt, S. (2007). "The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa" (PDF). Bulletin of the American Museum of Natural History. 302: 84. doi:10.1206/0003-0090(2007)302[1:taoeoa]2.0.co;2. 
  10. ^ 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

External links

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