Rauisuchia
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Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters) Triassic archosaurs. Originally it was believed that they were related to Erythrosuchids (Sill, 1974), but it is now known that they are crurotarsans (Benton 2004). Three families are generally recognised: Prestosuchidae, Rauisuchidae, and Poposauridae, as well as a number of forms (e.g. those from the Olenekian of Russia) that are too primitive and/or poorly known to fit in any of these groups. There has been considerable suggestion that the group as currently defined is paraphyletic, representing a number of related lineages independently evolving and filling the same ecological niche of medium to top terrestrial predator. For example, Parrish (1993) and Juul (1994) found poposaurid rauisuchians to be more closely related to Crocodilia than to prestosuchids. In a more recent study, Nesbitt (2003) presented a different phylogeny with a monophyletic Rauisuchia. The group may even be something of a "wastebasket taxon". Determining exact phylogenetic relationships is difficult because of the scrappy nature of a lot of the material. However, recent discoveries and studies such as those of Batrachotomus (Gower, 2002; Benton & Walker 2002) are shedding light on the evolutionary relationships of this poorly known but fascinating group.
José Bonaparte and following him Michael Benton argue (Bonaparte 1981, Benton, 1984) that rauisuchians such as Saurosuchus developed an erect stance independently of and differently to dinosaurs, by means of having the femur vertical and angling the acetabulum ventrally, rather than having an angled neck or curve in the femur. They refer to this as the pillar-erect posture.
The erect gait indicates that these animals were clearly active, agile predators, with locomotor superiority over the kannemeyerid dicynodonts and abundant rhynchosaurs on which they fed. They were successful animals, the largest with skulls a meter or more in length, and continued right until the end of the Triassic, when, along with many other large archosaurs, they were killed off by the end Triassic extinction event. With their demise, theropod dinosaurs were able to emerge as the sole large terrestrial predators; meat-eating dinosaur footprints suddenly increase in size at the start of the Jurassic, when rauisuchians are absent (Olsen et al. 2002).(Although, however, recently found Cretaceous fossils that were originally thought to be those of crocodiles, but resemble rauisuchians, might suggest that a few rauisuchians survived under the shadows of dinosaurs until the K-T Event.)
Well-known Rauisuchians include Ticinosuchus of the Middle Triassic of Europe (Switzerland and Northern Italy), Saurosuchus of the late Triassic (Late Carnian) of South America (Argentina), and Postosuchus of the late Triassic (Late Carnian to Early Norian) of North America (SW USA). One Rauisuchian, Teratosaurus, was for a long time even considered an early theropod dinosaur!
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[edit] References
- Benton, MJ (1984), Rauisuchians and the success of dinosaurs. Nature 310: 101
- Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd
- Benton, MJ & AD Walker (2002), Erpetosuchus, a crocodile-like basal archosaur from the Late Triassic of Elgin, Scotland, Zool. J. Linn. Soc. 136: 25-47.
- Bonaparte, JF (1984) Locomotion in rauisuchid thecodonts. J. Vert. Paleont. 3(4):210-218
- Carroll, RL (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
- Gower, DJ (2002), Braincase evolution in suchian archosaurs (Reptilia: Diapsida): evidence from the rauisuchian Batrachotomus kupferzellensis, Zool. J. Linn. Soc. 136: 49-76
- Juul, L (1994), The phylogeny of basal archosaurs. Palaeontologia Africana 31: 1-38
- Nesbitt, SJ (2003) Arizonasaurus and its implications for archosaur divergence. Proc. R. Soc. Lond. B (Suppl.) 270: S234-S237
- Olsen, P.E., Kent, D.V., H.-D.Sues,, Koeberl, C., Huber, H., Montanari, E.C.Rainforth, A., Fowell, S.J., Szajna, M.J., and Hartline, B.W., (2002) Ascent of Dinosaurs Linked to an Iridium Anomaly at the Triassic-Jurassic Boundary. Science, vol. 296, p. 1305-1307.
- Parrish, JM (1993), Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13: 287-308
- Sill, WD (1974). The anatomy of Saurosuchus galilei and the relationships of the rauisuchid thecodonts. Bull. Mus. Comp. Zool. 146: 317-362
