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eoraptor
''Eoraptor'' (pron.:"EE-oh-RAP-tor") was one of the world's earliest dinosaurs that lived ca. 231.4 million years ago,〔 ()〕 during the latter part of the Triassic Period in Western Gondwana, what is now the northwestern region of Argentina. It was a small sized ( long), lightly-built, ground-dwelling, two-legged bipedal saurischian, close to the ancestry of both theropods and sauropodomorphs.〔 It is known from several well-preserved skeletons. When first described in 1993, it was considered to be one of the earliest, if not the earliest known dinosaur. ''Eoraptor'' has heterodont dentition, suggesting that it was omnivorous, and that this feeding strategy had evolved early on in dinosaurs.
==Description==
''Eoraptor'' had a slender body that grew to about 1 meter (3.3 ft) in length, with an estimated weight of about 10 kilograms (22 lb). It has a lightly built skull with a slightly enlarged external naris.〔 Like the coelophysoids which would appear millions of years later, ''Eoraptor'' has a kink in its upper jaws, between the maxilla and the premaxilla. Sereno et al. (2013) observed that the lower jaw has a mid-mandibular joint.〔Paul C. Sereno, Ricardo N. Martínez & Oscar A. Alcober (2013) Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12: 83-179 DOI:10.1080/02724634.2013.820113〕 It ran digitigrade, and upright on its hind legs. The femur of the holotype specimen PVSJ 512 of ''Eoraptor'' is 152 mm, and the tibia is 157 mm, suggesting that it was a fast runner. Its forelimbs are only half the length of its hindlimbs, which would suggest that it was an obligate biped. All of its long bones have hollow shafts.〔 ''Eoraptor'' had five digits on each 'hand', the three longest of which ended in large claws and were presumably used to handle prey. Scientists have surmised that the fourth and fifth digits were too tiny to be of any use in hunting. The ilium is supported by three sacral vertebrae, unlike that of the coeval Herrerasaurus which is supported by only two sacrals, a basal trait.〔Langer, Max C. (2004). "Basal Saurischia". In Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 25–46. ISBN 0-520-24209-2.〕 ''Eoraptor'' had vertebral centra that are hollow, a feature present in some of its ancestors.
Bonaparte (1996) interpreted the relatively large orbital opening in the skull as a juvenile trait. Tykoski agreed (2005) and suggested that certain skull features of the type specimen suggested that it was young, specifically, the skull bones are not completely fused, relatively large orbits, and a short snout.〔Tykoski, 2005. Anatomy, ontogeny and phylogeny of coelophysoid theropods. PhD Dissertation. University of Texas at Austin. 553 pp.〕 Sereno et al. (1993), supported the notion that ''Eoraptor'' was an adult specimen based on the closure of sutures in the vertebral column, and the partial fusion of the scapulocoracoid.〔
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