Eoraptor (/ˈiːoʊˌræptər/) is agenus of small, lightly built,basalsauropodomorphdinosaur. One of the earliest-known dinosaurs and one of the earliest sauropodomorphs, it lived approximately 231 to 228million years ago,[1] during theLate Triassic in WesternGondwana, in the region that is now northwesternArgentina. Thetype 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 hadmultiple tooth shapes, which suggests that it wasomnivorous.
The bones of this primitive dinosaur were first discovered in 1991, byUniversity of San Juan paleontologist Ricardo Martínez, during field work conducted by theUniversity of Chicago and the University of San Juan. Theholotype specimenPVSJ 512 was discovered in muddysiltstone belonging to the Cancha de Bochas Member of theIschigualasto Formation in Argentina. The fossils in this formation were deposited in theCarnian stage of theTriassic period, approximately 235 to 228 million years ago. It took almost 12 months to collect the holotype,[citation needed] which was then shipped to theField 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 toSan Juan, Argentina, where it went on display at the Museum of Natural Sciences.
Skull cast
The genusEoraptor was described and named byPaul Sereno,Catherine Forster,Raymond R. Rogers, and Alfredo M. Monetta in1993.[2] The name is derived from the Greek wordeós (ἠώς) meaning 'dawn',[3] a reference to its primitive nature, and the Latin wordraptor meaning 'plunderer', a reference to its presumed carnivorous nature and its grasping hand. Thespecific namelunensis is derived from the Latin wordsluna ('moon') and the suffix-ensis ('inhabitant'), a reference to its place of discovery: theValle de la Luna ('Valley of the Moon'), so named for its arid, otherworldly appearance evocative of a lunar landscape. Thetype speciesEoraptor lunensis means 'dawn plunderer from the Valley of the Moon'.
Eoraptor compared in size to a humanSkeleton ofEoraptor lunensis, known remains depicted in white and light grey, unknown in dark grey
Eoraptor was a small dinosaur, with the known specimens measuring 1.2–1.3 m (3.9–4.3 ft) in length,[4] and weighing around or less than 10 kg (22 lb).[5] It had a lightly built skull with a slightly enlarged externalnaris.[4] As in earlysauropodomorphs such asBuriolestes andPampadromaeus andcoelophysoids (which would appear millions of years later),Eoraptor had a kink in its upper jaws, between themaxilla and thepremaxilla.Paul Serenoet al. (2013) observed that the lower jaw had a mid-mandibular joint.[4] It randigitigrade, and upright on its hind legs. Thefemur of theholotype specimen PVSJ 512 is 152 millimetres (6.0 in), and thetibia is 157 millimetres (6.2 in), suggesting that it was a fast runner. Its forelimbs are only half the length of its hindlimbs, suggesting that it wasbipedal. All of its long bones have hollow shafts.[4]Eoraptor had five digits on each 'hand', the three longest of which ended in largeclaws and were presumably used to handleprey. Scientists have surmised that the fourth and fifth digits were too tiny to be of any use in hunting. Theilium is supported by three sacral vertebrae (atypical of the plesiomorphic two sacrals of basal sauropodomorphs[6]), unlike that of the coevalHerrerasaurus which is supported by only twosacrals, abasal trait.[7]Eoraptor hadvertebral centra that are hollow, a feature present in some of its ancestors.
Restoration
The original describers,Paul Sereno et al. (1993), supported the notion thatEoraptor was an adult specimen based on the closure of sutures in thevertebral column, and the partial fusion of thescapulocoracoid.[2]Bonaparte (1996) interpreted the relatively largeorbital opening in the skull as a juvenile trait. Ronald Tykoski agreed (2005) and suggested that certain skull features of thetype specimen suggested that it was young, specifically, the skull bones are not completely fused, relatively large orbits, and a short snout.[8] 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.[4]
According to Serenoet al. (1993),Eoraptor can be distinguished based on the fact that itspremaxillary and anteriormaxillary teeth areleaf-shaped, the externalnares are slightly enlarged, and the premaxilla is observed to have a slender posterolateral process.[2] Max Langer andMichael Benton (2006) noted thatEoraptor can be distinguished based on the fact that the proximal part of itsfibula is extremely transversely compressed.[9]
In 1993Paul Sereno and his colleagues described and namedEoraptor, and determined it to be one of the earliest dinosaurs.[2][10] 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 specializedpredatory features. In 1995, Sereno posited thatEoraptor is the earliest-recordedtheropod, and is closest to "the hypothetical dinosaurian condition than any other dinosaurian subgroup."[11] The precise placement ofEoraptor within Dinosauria has been unstable, with opinion often varying between a basal saurischian and a basal theropod.[12] 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.[2] In 2011, a study conducted byHans-Dieter Sues,Sterling J. Nesbitt, David S. Berman and Amy C. Henrici featuring a description ofDaemonosaurus, also concluded that there is now enough fossil evidence to confidently classifyEoraptor as a theropod.[13] The study noted that the "transitional suite of character states" of the recently discovered dinosaurs,Daemonosaurus andTawa further support thatEoraptor is a basal theropod, and not a basal saurischian or a basal sauropodomorph.[14] On the other hand, several studies from 2012 onward have recoveredEoraptor as an early sauropodomorph, rather than a theropod.[4][15][16][17][18][19][20][21][22][23][24][25][excessive citations] The following phylogenetic tree illustrates the relationships ofEoraptor among the major theropod groups based on various studies conducted in the 2010s.[26]
Philip Currie (1997) foundEoraptor anatomically closer to what would be considered the ancestralmorphotype of bothsaurischian andornithischian dinosaurs.[27] In 2011, Martinezet al. (the team that describedEodromaeus) foundEoraptor to be a basal sauropodomorph, with characteristic features from the group.[28][29] Michael Benton expressed his hesitation to this, and claimed that it is "quite a shift" to removeEoraptor from Theropoda and then place it in Sauropodomorpha.[29] A subsequent study by Apaldetti, Martinez, Alcober, and Pol published in 2011 foundEoraptor to be a saurischian close to sauropodomorphs and theropods, though was unable to resolve which of the two branches, if either, it fell within.[30] Serenoet al. (2013) redescribed the holotype skeleton and concluded thatEoraptor was not a theropod but a basal sauropodomorph, consistent with the earlier observation made by Martinezet al. (2011).[4]
A large phylogenetic analysis of early dinosaurs by Matthew Baron,David Norman and Paul Barrett (2017) foundEoraptor to be the earliest diverging member ofTheropoda, within the larger cladeOrnithoscelida.[31] A phylogenetic analysis published with the description of newBuriolestes remains in 2018, based on Langeret al. (2017)[32] placedEoraptor in a clade of early sauropodomorphs, alongsideBuriolestes,Panphagia,Pampadromaeus, andSaturnalia.[33]
Eoraptor is thought to have been an omnivore,[4] although itsdentition is quite similar to that ofBuriolestes, which is considered carnivorous.[15][16] 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 ofEoraptor 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.[2]Eoraptor had 4 teeth in thepremaxilla and 18 teeth in themaxilla, adental formula not dissimilar to that ofHerrerasaurus.
During the Late Triassic period, the Ischigualasto Formation was a volcanically activefloodplain covered by forests, with a warm and humid climate,[34] but subject to seasonal variations including strong rainfall.[35]Vegetation consisted offerns,horsetails, and giantconifers, which formedhighland forests along the banks of rivers.[36]Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation.[37] Sereno (1993) noted thatEoraptor was found in "close association" withtherapsids,rauisuchians,archosaurs,Saurosuchus and the dinosaursHerrerasaurus andPisanosaurus, all of whom lived in its paleoenvironment.Herbivores were represented byrhynchosaurs such asHyperodapedon;aetosaurs;cynodonts likeChiniquodon,kannemeyeriiddicynodonts such asIschigualastia; andtraversodontids such asExaeretodon. These non-dinosaurian herbivores were much more abundant than early dinosaurs.[38] Dinosaur fossils, including those ofEoraptor only represent approximately 6% of the total sample that has been recovered from the Ischigualasto Formation (Rogerset al., 1993), which suggests that dinosaurs were less numerous than othertetrapods.[39]
^abcdefghSereno, Paul C.; Martínez, Ricardo N.; Alcober, Oscar A. (2013). "Osteology ofEoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina".Journal of Vertebrate Paleontology Memoir.12:83–179.doi:10.1080/02724634.2013.820113.S2CID86006363.
^Rauhut, O.W.M.; Fechner, R.; Remes, K.; Reis, K. (2011). "How to Get Big in the Mesozoic: The Evolution of the Sauropodomorph Body Plan". In Klein, N.; Remes, K.; Sander, P.M. (eds.).Biology of the Sauropod Dinosaurs: Understanding the Life of Giants. Indiana University Press. pp. 119–149.ISBN978-0253355089.
^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
^abMüller, Rodrigo T; Langer, Max C; Bronzati, Mario; Pacheco, Cristian P; Cabreira, Sérgio F; Dias-da-Silva, Sérgio (2018-05-15). "Early evolution of sauropodomorphs: anatomy and phylogenetic relationships of a remarkably well-preserved dinosaur from the Upper Triassic of southern Brazil".Zoological Journal of the Linnean Society.doi:10.1093/zoolinnean/zly009.ISSN0024-4082.
^Hendrickx, C.; Hartman, S.A.; Mateus, O. (2015). "An Overview of Non-Avian Theropod Discoveries and Classification".PalArch's Journal of Vertebrate Palaeontology.12 (1):1–73.
^Currie, P.J. (1997). Theropoda. InEncyclopedia of Dinosaurs (P.J. Currie and K. Padian, eds.) pp 731–736. Academic Press, San Diego, California.
^Müller, Rodrigo T.; Langer, Max C.; Bronzati, Mario; Pacheco, Cristian P.; Cabreira, Sérgio F.; Dias-da-Silva, Sérgio (15 May 2018). "Early evolution of sauropodomorphs: anatomy and phylogenetic relationships of a remarkably well-preserved dinosaur from the Upper Triassic of southern Brazil".Zoological Journal of the Linnean Society.184 (4):1187–1248.doi:10.1093/zoolinnean/zly009.S2CID90215853.
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