Eoraptor (jan 1, 231000000 BC – jan 1, 228000000 BC)

Description:

Eoraptor (/ˈiːoʊˌræptər/) is a genus of small, lightly built, basal saurischian dinosaurs. One of the earliest-known dinosaurs, it lived approximately 231 to 228 million years ago, during the Late Triassic in Western Gondwana, in the region that is now northwestern Argentina. The type and only species, Eoraptor lunensis, was first described in 1993, and is known from an almost complete and well-preserved skeleton and several fragmentary ones. Eoraptor had multiple tooth shapes, which suggests that it was omnivorous. Eoraptor had a slender body that grew to about 1 metre (3 ft 3 in) in length, with an estimated weight of about 10 kilograms (22 lb). It had a lightly built skull with a slightly enlarged external naris. As in early sauropodomorphs such as Buriolestes and Pampadromaeus and coelophysoids (which would appear millions of years later), Eoraptor had a kink in its upper jaws, between the maxilla and the premaxilla. Paul Sereno et al. (2013) observed that the lower jaw had a mid-mandibular joint. It ran digitigrade, and upright on its hind legs. The femur of the holotype specimen PVSJ 512 is 152 millimetres (6.0 in), and the tibia is 157 millimetres (6.2 in), suggesting that it was a fast runner. Its forelimbs are only half the length of its hindlimbs, suggesting that it was bipedal. 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 (atypical of the plesiomorphic two sacrals of basal sauropodomorphs), unlike that of the coeval Herrerasaurus which is supported by only two sacrals, a basal trait. 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. Ronald 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. Paul 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. According to Sereno et al. (1993), Eoraptor can be distinguished based on the fact that its premaxillary and anterior maxillary teeth are leaf-shaped, the external nares are slightly enlarged, and the premaxilla is observed to have a slender posterolateral process. Max Langer and Michael Benton (2006) noted that Eoraptor can be distinguished based on the fact that the proximal part of its fibula is extremely transversely compressed.

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