Eoraptor was a genus of small, slender theropod native to northwest Argentina. It was quite possibly the earliest theropod genus and has not been classified in any family. It may be a ceratosaur, or perhaps a herrerrosaurid; however, it may also be more primitive than that. Likely, Eoraptor was a basal theropod, in a family of its own; it or an undiscovered relative may in fact be the ancestor to all more advanced theropods. The type and only species has been named Eoraptor lunensis, meaning "dawn robber from the Valley of the Moon". Although traditionally it has been classified as a theropod, it may in fact be a basal archosaur, representing the common ancestor of all dinosaur species.
Discovery and fossils
Eoraptor fossils 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.
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 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.
Eoraptor was a typical early dinosaur, sharing several features with other primitive dinosaur genera that have been discovered. Eoraptor had long, slender legs ending in splayed feet, long front arms with five fingers, a long, tapering tail, a long neck ending in a small head, and a deep midsection. Like all early dinosaur species, it was slender in build, and would have had a good sense of sight ; its large eyes would have helped it to see in darkness, which may suggest that it was nocturnal or lived in dense rainforests. Its teeth were not specialized for hunting, nor for feeding on plants, as described above. This means that like some later theropods, including Bahariasaurus, Eoraptor would have been a generalist, in its case probably feeding on insects as well as plant matter.
All of the early dinosaurs discovered as of yet were fairly small; Eoraptor was no exception. It was one of the smallest non-avian theropods, with an estimated weight of 22 pounds (10 kilograms), similar in weight to a goose or swan. In length, Eoraptor would have averaged around 1 meter (3 feet) in length, again similar to a swan in terms of size.