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Letters to Nature
Nature 361, 64 - 66 (07 January 1993); doi:10.1038/361064a0

Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria

Paul C. Sereno*, Catherine A. Forster*, Raymond R. Rogers & Alfredo M. Monetta

*Department of Organismal Biology and Anatomy, University of Chicago, 1027 East 57th Street, Chicago, Illinois 60637, USA
Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA
Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan 5400, Argentina

WE report here the discovery of a primitive dinosaur skeleton from Upper Triassic strata in northwestern Argentina. The 1-m-long skeleton represents a new taxon, Eoraptor lunensis gen. et sp. nov., which is close to the predicted structure and size of the common dinosaurian ancestor1–5. The skull, which has a unique heterodont dentition, does not exhibit any of the specializations of the major dinosaurian clades (Ornithischia, Sauropodomorpha, Theropoda). The forelimbs are less than half the length of the hind limbs, which suggests an obligatory bipedal posture. Although close in overall form to the common dinosaurian ancestor, the functionally tridactyl, grasping/raking hand and other features show that Eoraptor is allied phylogenetically with theropods. The discovery of Eoraptor supports the hypothesis that dinosaurs diverged rapidly at small body size from a common ancestor, with the principal herbivorous and carnivorous lineages present by the middle Carnian.

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