Rauisuchians and the Success of Dinosaurs

N:..::.A:.:.TIJRE=;_;V_::.OL:;;.·;;;..:3!0~12...:.JU;;..:L_Y_I984____________ NEWSANDVIEWS------------------1=01 Vertebrate palaeontology of earlier dominant groups (mammal-like reptiles, rhynchosaurs, thecodontians). Further weight is given to this view by the Rauisuchians and the success fact that rauisuchians had an erect gait and yet were replaced by the dinosaurs. Dinosaurs and rauisuchians achieved an of dinosaurs erect gait in different ways. In a primitive from Michael Benton sprawling reptile, the thigh bone (femur) sticks out roughly sideways, and the head THROUGHOUT most of the Triassic period example, keyhole-shaped antorbital fen of the femur fits straight into the sideways (245-208 Myr), and before the age of the estra, movable joints and extra slot-like facing bowl-shaped pelvic socket. In an dinosaurs, the top carnivores were a group fenestra between the premaxilla and max erect animal, the femur points straight of reptiles called rauisuchians. Although illa) and pelvis (acetabulum faces ventrally, down, and changes have to occur at the this group was formerly thought to have low ilium, pubis antero-ventral to ilium). joint between femur and pelvis. In dino had a restricted distribution, recent papers There are two families, the quadrupedal saurs (and in mammals), the femur prove that its distribution was in fact Rauisuchidae and the bipedal Popo develops a head at right angles to its shaft worldwide 1•8 • More provocatively, some of sauridae. A typical late Triassic rauisuchid, (Fig.la). In rauisuchians, on the other the new work suggests the need to modify Saurosuchus (Fig. I), was 6-7 m long, with hand, the pelvis is tipped over, so that the commonly held views about both the a massive skull, short neck, fairly long pelvic socket faces downwards instead of evolution of reptiles in the Triassic and the limbs and a powerful tail. sideways (Fig.lb). Recently, it has become origin of the dinosaurs. What is of more importance, from the evident that erect gait was also achieved by The Rauisuchia were archosaurs (the point of view of archosaur evolution in the a third group of archosaurs - the salto group of reptiles that also includes dino Triassic and, in particular, theories about posuchid crocodilomorphs, distant rela 14 saurs and crocodiles) and belonged to the the origin and success of the dinosaurs, is tives of the living crocodiles • In the late basal group, the 'Thecodontia'. Several of new work showing rauisuchians had an Triassic, then, three archosaur lineages 2 3 them- such as Poposaurus, Teratosaurus erect gait • • The standard view is that ad showed independently derived advances in and numerous unnamed forms that have vances in the limb posture of archosaurs locomotion, and yet only the dinosaurs been identified on the basis of isolated during the Triassic explain why dinosaurs survived - the other two lineages died out teeth and jaws2.3 - were formally were so successful. The early Triassic bytheendoftheTriassic. 0 classified as dinosaurs. In fact, a whole archosaurs had a primitive sprawling I. Chatterjee, S. Nature 295, 317 (1982). assemblage of fictitious carnivorous dino posture, like that of living lizards and sala 2. Bonaparte, J.F. Rev. Mus. Argent. Cienc. nat. Paleont. 3, saurs - the Teratosauridae - was found manders. The posture of archosaurs sup ss (1981). 3. Bonaparte, J .F. J. vert. Paleont. 3, 210 (1984). ed upon middle and late Triassic rauisuch posedly became semi-erect in the middle 4. Ochev, V.G.Paleont. J.191l,210(1982). ian teeth and the skeletons ofprosauropod Triassic, and finally dinosaurs, with their S. Pinna, G. &.Arduini, P. Natura. Milano 69,73 (1978). 9 10 6. Wild, R. Mesozoic vert. Life 1, IS (1980). dinosaurs • • The oldest known true limbs tucked right under the body, emerg 7. Dawley, R.M., Zawiskie, J.M. &. Cosgriff, J.W. J. 11 dinosaurs occurred in the middle of the late ed in the Triassic • Paleont. 53, 1428 (1979). Triassic (about 225 Myr), and would have 8. Dutuit, J.-M. Annlspoteont. vert. 65, SS (1979). The erect posture is commonly thought 9. Walker, A.D. Phil. Trans. R. Soc. B:WS, 53 (1964). been preyed upon by rauisuchians. It has to have allowed increased running speed, 10. Benton, M.J. Abstr. Jrd Symp. Mesozoic terrest. Ecosyst. been suggested that rauisuchians were agility and size, allowing these animals to (in the press). 1 11. Charig, A.J. in Studies in Vertebrate Evolution (eds Joysey, ancestral to (some) dinosaurs , but that is outcompete all manner of other tetrapods K.A. & Kemp, T.S.) 121 (Oliver& Boyd, Edinburgh, 1972). 12 unlikely because of their specialized in the middle and late Triassic • However, 12. Charig, A.J. Mem. Soc. gtol. Fr. 139, 207 (1980); Symp. 13 zoot. Soc. Lond. 51 (in the press). ankles, pelvis and skull. The rauisuchians as I have previously argued , large-scale 13. Benton, M.J. Q. Rev. Bioi. 511, 29 (1983); Nature 301, 16 died out at the end of the Triassic without 'competition' of this kind between high (1983). leaving any descendants, that is as far as we level taxonomic groups and spread over 14. Crush, P.J. Paleontology 27, 131 (1984). can tell. tens of millions of years is most unlikely. Rauisuchians are characterized by More probably, the dinosaurs radiated Michael Benton is at the University Museum, several derived characters of the skull (for opportunistically only after the extinction Parks Road, Oxford OXJ 3PW. Fig. I Rauisuchians, giant meat-eaters of the Triassic. a, b, Hind limbs and pelvis of an early 'dinosaur' (a) and Saurosuchus (b) viewed from behind. c, Saurosuchus from the late Triassic of Argentina, 6-7m long. Both dinosaurs and rauisuchians had erect gait, but this advance was achieved in different ways in each group: a bent head on the femur in dinosaurs (a) and a horizontal pelvic socket in rauisuchians (b). From refs 2,3. 10 em © 1984 Nature Publishing Group.

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