Eoraptor was one of the world's earliest dinosaurs. It was a two-legged carnivorous theropod  that lived around 228 million years ago, in what is now the northwestern region of Argentina. The type species is Eoraptor lunensis, which means 'dawn plunderer from the Valley of the Moon', denoting where it was originally discovered (Greek eos/ηως meaning 'dawn' or 'morning' and Latin lunensis meaning 'of the moon'). Paleontologists believe the Eoraptor resembles the common ancestor of all dinosaurs. It is known from several well-preserved skeletons.
Eoraptor compared in size to a human.It had a thin body that grew to about 1 meter (3 ft) in length, with an estimated weight of about 10 kilograms (22 lb). It ran digitigrade, upright on its hind legs. Its fore limbs were only half the length of its hind limbs and it had five digits on each 'hand'. Three of those digits, the longest of the five, 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.
Eoraptor is thought to have eaten mostly small animals. It was a swift sprinter and, upon catching its prey, it would use claws and teeth to tear the prey apart. However, it had both carnivore-type and herbivore-type teeth, so it could possibly have been omnivorous.
Artist's impressionThe bones of this primitive dinosaur were first discovered in 1991, by University of San Juan paleontologist Ricardo Martínez, during field work conducted by the University of Chicago and the University of San Juan in the Ischigualasto Basin of Argentina. During the Late Triassic Period, this was a river valley but is now desert badlands. Eoraptor was found in the Ischigualasto Formation, the same formation that yielded Herrerasaurus, a very early theropod. By 1993 paleontologist Paul Sereno determined it to be one of the earliest dinosaurs. Its age was determined by several factors, not least because it lacked the specialised features of any of the major groups of later dinosaurs, including its lack of specialized predatory features. Unlike later carnivores, it lacked a sliding joint at the articulation of the lower jaw, with which to hold large prey. Furthermore, only some of its teeth were curved and saw-edged, unlike those in a later predator's mouth.
Skull castThe fact that it possessed some herbivore teeth and five fully developed 'fingers' has led scientists to place Eoraptor at more ancient than even Herrerasaurus. There is a possibility that Staurikosaurus may be older, but it is rather large. Staurikosaurus seems to have features in common with prosauropods, which has led scientists to question how primitive Eoraptor was in relation to other dinosaurs, but Nesbitt et al. (2009) found it to be a theropod more derived than herrerasaurids but more basal than Coelophysis.