Crurotarsi

Crurotarsi is a group of archosauriform reptiles that includes the archosaurs (represented today by birds and crocodilians) and the extinct, crocodile-like phytosaurs.[1] Prior to 2011, the name had instead been used to refer to archosaurs closer to crocodilians than to birds and other dinosaurs. Another common term for croc-line archosaurs is Pseudosuchia. The pre-2011 definition of Crurotarsi assumed that phytosaurs were closer to crocodilians than to birds. However, a 2011 study argued that phytosaurs were not crown-archosaurs (i.e. their lineage evolved prior to the split between bird-line and croc-line archosaurs). Crurotarsi was redefined to pertain to a larger group which included Phytosauria and Archosauria (true crown-archosaurs).[1]

Crurotarsans
Temporal range:
Early Triassic–Present, 250–0 Ma
Life restoration of Protome batalaria
Life restoration of Ornithosuchus woodwardi
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauriformes
Clade: Crurotarsi
Sereno & Arcucci, 1990
Subgroups

The name Crurotarsi is derived from the Latin word crus (lower leg) and the Greek word tarsos (ankle). It refers to the specialized articulation (a crurotarsal joint) between the lower leg (specifically the fibula) and the ankle (specifically the calcaneum) which is present in the skeletons of reptiles such as suchians and phytosaurs. In their ankle joint, a hemicylindrical condyle on the calcaneum articulates into a concave area on the 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 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".[1] 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).[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. 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. hdl:2246/6112. S2CID 83493714.
  2. 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. 11 (Suppl. 4): 1–51. doi:10.1080/02724634.1991.10011426.
  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. S2CID 86687464.
  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.; 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 crocodile 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: Biological Sciences. 273 (1590): 1045–1048. doi:10.1098/rspb.2005.3426. PMC 1560254. PMID 16600879.
  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. hdl:2246/5840.
  10. Brusatte, Stephen L.; Benton, Michael J.; Desojo, Julia B.; Langer, Max C. (2010). "The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida)" (PDF). Journal of Systematic Palaeontology. 8 (1): 3–47. doi:10.1080/14772010903537732. S2CID 59148006.
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