Temporal range: Triassic - Recent, 250–0 Ma
Examples of extinct crurotarsans. Clockwise from top-left: Longosuchus meani (an aetosaur), Angistorhinus grandis, (a phytosaur), Saurosuchus galilei (a rauisuchian), Pedeticosaurus leviseuri (a sphenosuchian), Chenanisuchus lateroculi (a dyrosaurid), and Dakosaurus maximus (a thalattosuchian). Three species of living crurotarsan: Gharial (left), American Alligator (center), and American Crocodile (right). Scientific classification Kingdom: Animalia Phylum: Chordata Class: Reptilia Division: Archosauria Node: Crurotarsi
Sereno & Arcucci, 1990
The Crurotarsi (name derived from the Latin words crus and tarsus; 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) are a group of archosauriformes, represented today by the crocodiles, alligators, and gharials (and possibly birds) 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, but with a different definition. Crurotarsi include, by most published definitions, all descendants of the common ancestor of modern crocodiles, ornithosuchids, aetosaurs, and phytosaurs. According to two studies published in 2011, this definition would also include all other true archosaurs as well, due to the possibly very primitive position of the phytosaurs. A more restrictive group defined as all archosaurs closer to crocodiles than to birds (matching the traditional content of Crurotarsi) is the Pseudosuchia.
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.
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 armored 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.
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 are 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.
It is possible that crocodile-line archosaurs could one day be found that are outside Crurotarsi, not being descendants of the most recent common ancestor of phytosaurs, ornithosuchids, and suchians. These animals would still be pseudosuchians because they are more closely related to crocodiles than to birds. Brochu (1997) hypothesized that there could be non-crurotarsan pseudosuchians from the Late Permian, or that a newly discovered bird-line archosaur from the Late Permian could imply a ghost lineage of pseudosuchians into the Permian. Currently, all known archosaurs are Triassic or younger. Crurotarsans must have first appeared in the Triassic because the group is defined by taxa that are Triassic or younger. Under its current definition, Crurotarsi cannot include Permian archosaurs unless Permian phytosaurs, ornithosuchids, or suchians are discovered.
The scope of Crurotarsi has recently been changed by the phylogenetic placement of phytosaurs. 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, crurotarsans are not solely crocodile-line archosaurs but also bird-line archosaurs and phytosaurs. Under this phylogeny, Crurotarsi includes phytosaurs, crocodiles, pterosaurs, dinosaurs, and birds, while Pseudosuchia still contains only crocodile-line archosaurs. Below is a cladogram modified from Nesbitt (2011) showing the new changes:
Avemetatarsalia (bird-line archosaurs)
Pseudosuchia (crocodile-line archosaurs)
Archosauriformes Archosauria Crurotarsi
Rauisuchia "Group X" "Group Y" or Shuvosaurinae
Cladogram after Brusatte, Benton, Desojo and Langer (2010) 
Archosauriformes Archosauria Crurotarsi
Paracrocodylomorpha Bathyotica Rauisuchia Rauisuchoidea
- ^ Sereno, Paul (1991). "Basal archosaurs: phylogenetic relationships and functional implications". Journal of Vertebrate Paleontology (Suppl.) 11: pp. 1–51.
- ^ a b c Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades". Bulletin of the American Museum of Natural History 352: 1–292. doi:10.1206/352.1. http://digitallibrary.amnh.org/dspace/bitstream/2246/6112/1/B352.pdf.
- ^ a b 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.
- ^ Gauthier, J. A., Nesbitt, S. J., Schachner, E. R., Bever, G. S., and W. G. Joyce. (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.
- ^ Gauthier, J.A.; and 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.
- ^ Benton, M.J.; and 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.
- ^ a b 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.
- ^ 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.
- ^ 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.
- ^ 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. PMC 2872409. PMID 20308590. http://www.pnas.org/content/early/2010/03/15/1001706107.
Archosauromorphs Kingdom: Animalia · Phylum: Chordata · Class: Sauropsida · Subclass: Diapsida Primitive
Crurotarsi Archosaurs Avemetatarsalia and
Avian Archosaurs Kingdom: Animalia · Phylum: Chordata · Class: Sauropsida · Infraclass: Archosauromorpha Major cladesRelated article Plant-eating generaLotosauridaeOthers Bipeds or facultatively bipedal generaYarasuchusOthers Semi-aquatic or aquatic generaOthers Quadrupedal terrestrial carnivoresOthers
Wikimedia Foundation. 2010.