Eoraptor

Eoraptor () 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.
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 belonging to 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 "Eoraptor" was described and named by Paul Sereno, Catherine Forster, Raymond R. Rogers, and Alfredo M. Monetta in 1993. The name is derived from the Greek word () meaning 'dawn', a reference to its primitive nature, and the Latin word meaning 'plunderer', a reference to its presumed carnivorous nature and its grasping hand. The specific name "lunensis" is derived from the Latin words ('moon') and the suffix ('inhabitant'), a reference to its place of discovery: the ('Valley of the Moon'), so named for its arid, otherworldly appearance evocative of a lunar landscape. The type species "Eoraptor lunensis" literally means 'dawn plunderer from the Valley of the Moon'.
Description.
"Eoraptor" had a slender body that grew to about in length, with an estimated weight of about . 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 , and the tibia is , 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, 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.
Classification.
In 1993 Paul Sereno and his colleagues described and named "Eoraptor", 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 anatomical features. In 2011, a study conducted by Hans-Dieter Sues, Sterling J. Nesbitt, David S. Berman and Amy C. Henrici featuring a description of "Daemonosaurus", also concluded that there is now enough fossil evidence to confidently classify "Eoraptor" as a theropod. The study 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. On the other hand, several studies from 2012 onward have recovered "Eoraptor" as an early sauropodomorph, rather than a theropod. The following phylogenetic tree illustrates the relationships of "Eoraptor" among the major theropod groups based on various studies conducted in the 2010s.
Philip Currie (1997) found "Eoraptor" anatomically closer to what would be considered the ancestral morphotype of both saurischian and ornithischian dinosaurs. In 2011, Martinez "et al." (the team that described "Eodromaeus)" found "Eoraptor" to be a basal sauropodomorph, with characteristic features from the group. Michael Benton expressed his hesitation to this, and claimed that it is "quite a shift" to remove "Eoraptor" from Theropoda 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).
A large phylogenetic analysis of early dinosaurs by Matthew Baron, David Norman and Paul Barrett (2017) found "Eoraptor" to be the earliest diverging member of Theropoda, within the larger clade Ornithoscelida. A phylogenetic analysis published with the description of new "Buriolestes" remains in 2018, based on Langer "et al." (2017) placed "Eoraptor" in a clade of early sauropodomorphs, alongside "Buriolestes", "Panphagia", "Pampadromaeus", and "Saturnalia".
Paleobiology.
"Eoraptor" is thought to have been an omnivore, although its dentition is quite similar to that of "Buriolestes", which is considered faunivorous. 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 the mouths of later theropods. The heterodont dentition of "Eoraptor" consists of both serrated, recurved teeth in the upper jaw, like the teeth of theropods, and leaf-shaped teeth in the 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".
Paleoecology.
During the Late Triassic period, the Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate, but subject to seasonal variations including strong rainfall. 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"; aetosaurs; cynodonts like "Probelesodon", kannemeyeriid dicynodonts such as "Ischigualastia"; and traversodontids 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.