Eoraptor
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Eoraptor (Template:IPAc-en) is a genus of small, lightly built, basal sauropodomorph dinosaur. One of the earliest-known dinosaurs and one of the earliest sauropodomorphs, it lived approximately 231 to 228 million years ago,<ref name="OARM2010">Template:Cite journal [1] Template:Webarchive</ref> 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 discoveryEdit
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,Template:Citation needed 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.<ref name=sereno93/> The name is derived from the Greek word {{#invoke:Lang|lang}} (Template:Wikt-lang) meaning 'dawn',<ref name="Liddell 1980">Template:Cite book</ref> a reference to its primitive nature, and the Latin word Template:Wikt-lang meaning 'plunderer', a reference to its presumed carnivorous nature and its grasping hand. The specific name lunensis is derived from the Latin words {{#invoke:Lang|lang}} ('moon') and the suffix Template:Wikt-lang ('inhabitant'), a reference to its place of discovery: the {{#invoke:Lang|lang}} ('Valley of the Moon'), so named for its arid, otherworldly appearance evocative of a lunar landscape. The type species Eoraptor lunensis means 'dawn plunderer from the Valley of the Moon'.
DescriptionEdit
Eoraptor was a small dinosaur, with the known specimens measuring Template:Cvt in length,<ref name=sereno13/> and weighing around or less than Template:Cvt.<ref>Template:Cite book</ref> It had a lightly built skull with a slightly enlarged external naris.<ref name=sereno13/> 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.<ref name=sereno13>Template:Cite journal</ref> It ran digitigrade, and upright on its hind legs. The femur of the holotype specimen PVSJ 512 is Template:Convert, and the tibia is Template:Convert, 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.<ref name=sereno13/> 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 (atypical of the plesiomorphic two sacrals of basal sauropodomorphs<ref>Template:Cite journal</ref>), unlike that of the coeval Herrerasaurus which is supported by only two sacrals, a basal trait.<ref>Langer, Max C. (2004). "Basal Saurischia". In Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 25–46. Template:ISBN.</ref> Eoraptor had vertebral centra that are hollow, a feature present in some of its ancestors.
The original describers, 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.<ref name=sereno93/> 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.<ref>Tykoski, 2005. Anatomy, ontogeny and phylogeny of coelophysoid theropods. PhD Dissertation. University of Texas at Austin. 553 pp.</ref> Later Sereno et al. (2013) considered the type specimen as a young adult approaching skeletal maturity, considering that it contained traits of both maturity and immaturity.<ref name=sereno13/>
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.<ref name=sereno93/> 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.<ref>Langer, M. C., and Benton, M. J., 2006, Early Dinosaurs: a phylogenetic study: Journal of Systematic Palaeontology, vol 4, n. 4, p. 309–358.</ref>
ClassificationEdit
In 1993 Paul Sereno and his colleagues described and named Eoraptor, and determined it to be one of the earliest dinosaurs.<ref name=sereno93/><ref name="Princeton_Paul_2010">Paul, G.S., The Princeton Field Guide to Dinosaurs (Princeton University Press, 2010), p. 68.</ref> 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."<ref>P. C. Sereno. 1995. Theropoda: early evolution and major patterns of diversification Journal of Vertebrate Paleontology 15(3, suppl.):52A-53A</ref> The precise placement of Eoraptor within Dinosauria has been unstable, with opinion often varying between a basal saurischian and a basal theropod.<ref>Template:Cite journal</ref> 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.<ref name=sereno93/> 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.<ref name="Daemonosaurus.2011.Sues&Bergman">Bergman, David S.; Sues, Hans-Dieter. (2011), "A late-surviving basal theropod dinosaur from the latest Triassic of North America", Proceedings of the Royal Society B, published online 13-4-2011.</ref> 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.<ref name=sues11>Hans-Dieter Sues, Sterling J. Nesbitt, David S. Berman and Amy C. Henrici (2011). "A late-surviving basal theropod dinosaur from the latest Triassic of North America" Proceedings of the Royal Society B 278 (1723): 3459–3464</ref> On the other hand, several studies from 2012 onward have recovered Eoraptor as an early sauropodomorph, rather than a theropod.<ref name="sereno13"/><ref name=":0">Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>Template:Excessive citations inline The following phylogenetic tree illustrates the relationships of Eoraptor among the major theropod groups based on various studies conducted in the 2010s.<ref name="theropodphylogeny2015">Template:Cite journal</ref>
Philip Currie (1997) found Eoraptor anatomically closer to what would be considered the ancestral morphotype of both saurischian and ornithischian dinosaurs.<ref>Currie, P.J. (1997). Theropoda. In Encyclopedia of Dinosaurs (P.J. Currie and K. Padian, eds.) pp 731–736. Academic Press, San Diego, California.</ref> In 2011, Martinez et al. (the team that described Eodromaeus) found Eoraptor to be a basal sauropodomorph, with characteristic features from the group.<ref>Template:Cite journal</ref><ref name="NNews_2011">Kaplan, M. "Move over Eoraptor" Template:Webarchive, http://www.nature.com/news, 13-1-2011.</ref> 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.<ref name="NNews_2011"/> 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.<ref name=Apaldetti2011>Template:Cite journal</ref> 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).<ref name=sereno13/>
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.<ref name="Ornithoscelida">Template:Cite journal</ref> A phylogenetic analysis published with the description of new Buriolestes remains in 2018, based on Langer et al. (2017)<ref>Template:Cite journal</ref> placed Eoraptor in a clade of early sauropodomorphs, alongside Buriolestes, Panphagia, Pampadromaeus, and Saturnalia.<ref name="Muller2018">Template:Cite journal</ref>
PaleobiologyEdit
Eoraptor is thought to have been an omnivore,<ref name="sereno13"/> although its dentition is quite similar to that of Buriolestes, which is considered carnivorous.<ref name=":0"/><ref name=":1"/> 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.<ref name="sereno93">Template:Cite journal</ref> Eoraptor had 4 teeth in the premaxilla and 18 teeth in the maxilla, a dental formula not dissimilar to that of Herrerasaurus.
PaleoecologyEdit
During the Late Triassic period, the Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate,<ref name="TuckerBenton1982">Template:Cite journal</ref> but subject to seasonal variations including strong rainfall.<ref name="Columbi2008">Template:Cite conference</ref> Vegetation consisted of ferns, horsetails, and giant conifers, which formed highland forests along the banks of rivers.<ref name ="Sereno1992">Template:Cite journal</ref> Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation.<ref>Template:Cite journal</ref> 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 Chiniquodon, kannemeyeriid dicynodonts such as Ischigualastia; and traversodontids such as Exaeretodon. These non-dinosaurian herbivores were much more abundant than early dinosaurs.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
ReferencesEdit
External linksEdit
- Template:Commons category-inline
- Template:Wikispecies-inline
- Paul Sereno's paleontology website
- Paleobiology Database entry
- Eoraptor lunensis at DigiMorph
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