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 probably ate 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.
The bones of this primitive dinosaur were first discovered in 1991, by University of Chicago paleontologist Paul Sereno, 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 it had been determined 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.
Eoraptor belonged to a major group of dinosaurs called saurischians, or lizard-hipped dinosaurs. Their hip structures are similar to that of the modern lizard.
The fact that it possessed some herbivore teeth and five fully developed 'fingers' has led scientists to place Eoraptor at more ancient than even Herrerasaurus. Only some prosauropods, recently discovered in Madagascar, are thought to be older. There is a possibility that Staurikosaurus may be older, but it is rather large. Staurikosaurus seems to have features in common with both prosauropods and theropods, which has led scientists to question how primitive Eoraptor was in relation to other dinosaurs.