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 had a lightly built skull with a slightly enlarged external naris. Like the coelophysoids which would appear millions of years later, Eoraptor had a kink in its upper jaws, between the maxilla and the premaxilla. 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 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 basaltrait. 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. 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.
In 1993 paleontologist Paul Sereno and colleagues described and named the species, and determined it to be one of the earliest dinosaurs. Its age was determined by several factors, not least because it lacked the specialized features of any of the major groups of later dinosaurs, including its lack of specialized predatory features. In 1995, Sereno posited that Eoraptor is the earliest recorded theropod, and is closest to "the hypothetical dinosaurian condition than any other dinosaurian subgroup." The precise placement of Eoraptor within Dinosauria has been unstable, with opinion often varying between a basal saurischian and a basal theropod. When it was first described by Sereno and Forster in 1993, it was regarded as a theropod, based on its "functionally tridactyl hand" and other features. In 2011, a study by Sues, Nesbitt, Berman and Henrici featuring description of Daemonosaurus, also concluded that there is now enough fossil evidence to confidently classify Eoraptor as a theropod. Sues et al. noted that the "transitional suite of character states" of the recently discovered dinosaurs, Daemonosaurus and Tawa further support that Eoraptor is a basal theropod, and not a basal saurischian or a basal sauropodomorph. The following family tree illustrates a synthesis of the relationships of the major theropod groups based on various studies conducted in the 2010s, showing the current consensus position of Eoraptor.
Currie (1997) found Eoraptor anatomically closer to what would be considered the ancestral morphotype of both saurischian and ornithischian dinosaurs. In 2011, as part of the team that described Eodromaeus, Martinez and others found Eoraptor to be a basal sauropodomorph, with characteristic features for that group. Michael Benton has expressed his hesitation to this, and claims that it is "quite a shift" to remove Eoraptorfrom the theropods and then place it in Sauropodomorpha. A subsequent study by Apaldetti, Martinez, Alcober, and Pol published in 2011 found Eoraptor to be a saurischian close to sauropodomorphs and theropods, though was unable to resolve which of the two branches, if either, it fell within. Sereno et al. (2013) redescribed the holotype skeleton and concluded that Eoraptor was not a theropod but a basal sauropodomorph, consistent with the earlier observation made by Martinez et al. (2011).
Distinguishing anatomical features
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.
According to Sereno et al. (1993), Eoraptor can be distinguished based on the fact that its premaxillary and anterior maxillaryteeth are leaf-shaped, the external nares are slightly enlarged, and the premaxilla is observed to have a slender posterolateral process. Langer and Benton (2006) note that Eoraptor can be distinguished based on the fact that the proximal part of its fibula is extremely transversely compressed.
Eoraptor is thought to have been an omnivore. It was a swift sprinter and, upon catching its prey, it would use claws and teeth to tear the prey apart. Unlike later, carnivorous dinosaurs, 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 unique heterodont dentition of Eoraptor (Sereno et al., 1993) consists of both serrated, recurved teeth in the maxillae (upper jaws), like the teeth of theropods, and leaf-shaped teeth in the dentary (lower jaw), like the teeth of basal sauropodomorphs. Eoraptor had 4 teeth in the premaxilla and 18 teeth in the maxilla, a dental formula not dissimilar to that of Herrerasaurus. Tweet (2003) noted that the variability observed in the teeth (heterodonty) of Eoraptor, suggests that it descended from ancestors that were omnivorous, or it was itself omnivorous.
History of discovery
The 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. The holotype specimen PVSJ 512 was discovered in muddy siltstone from the Cancha de Bochas Member of the Ischigualasto Formation in Argentina. The fossils in this formation were deposited in the Carnian stage of the Triassic period, approximately 235 to 228 million years ago. It took almost 12 months to collect the holotype, which was then shipped to the Field Museum of Natural History in Chicago for preparation by William F. Simpson and Bob Masek. The fossil was first put on display in Chicago and was then returned to San Juan, Argentina, where it went on display at the Museum of Natural Sciences.
The genus name Eoraptor is derived from the Greek word "eos" (ηως) meaning "dawn", a reference to its primitive nature and the Latin word "raptor" meaning "plunderer", a reference to its grasping hand. The species name, "lunensis" is derived from the Latin words, "luna" meaning "moon", and the suffix "-ensis", meaning "inhabitant". The species name lunensis is a reference to its place of discovery the "Valle de la Luna", which is Spanish for "Valley of the Moon." This valley is so named because of its arid, otherworldly appearance, evocative of a lunar landscape. The type species is Eoraptor lunensis, which means "dawn plunderer from the Valley of the Moon". Eoraptor was described and named by Paul C. Sereno, Catherine A. Forster, Raymond R. Rogers, and Alfredo M. Monetta in 1993.
During the Late Triassic Period, the Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate, though subject to seasonal variations including strong rainfalls. Vegetation consisted of ferns, horsetails, and giant conifers, which formed highland forests along the banks of rivers. Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation. Sereno (1993) noted that Eoraptor was found in "close association" with therapsids, rauisuchians, archosaurs, Saurosuchus and the dinosaurs Herrerasaurus and Pisanosaurus, all of whom lived in its paleoenvironment. Herbivores were represented by rhynchosaurs such as Hyperodapedon (a beaked reptile); aetosaurs (spiny armored reptiles); cynodontslike Probelesodon, kannemeyeriid dicynodonts (stocky, front-heavy beaked quadrupedal animals) such as Ischigualastia; and traversodontids (somewhat similar in overall form to dicynodonts, but lacking beaks) such as Exaeretodon. These non-dinosaurian herbivores were much more abundant than early dinosaurs. Dinosaur fossils, including those of Eoraptor only represent approximately 6% of the total sample that has been recovered from the Ischigualasto Formation (Rogers et al., 1993), which suggests that dinosaurs were less numerous than other tetrapods.