Temporal range: Late Triassic
|Reconstructed skeleton, Royal Ontario Museum|
Sereno et al., 1993
Sereno et al., 1993
Eoraptor (//) was one of the earliest known dinosaurs, living approximately 231 to 228 million years ago, during the Late Triassic in Western Gondwana, in the region that is now northwestern Argentina. It was a small, lightly-built, basal theropod dinosaur. It is known from several well-preserved skeletons. When first described in 1993, it was considered to be one of the earliest, if not the earliest known dinosaur. Eoraptor has heterodont dentition, suggesting that it was omnivorous, and that this feeding strategy had evolved early on in dinosaurs.
Eoraptor had a slender body that grew to about 1 metre (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. 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 152 millimetres (6.0 in), and the tibia 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 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.
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 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.
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 presumed carnivorous nature and its grasping hand. The specific name lunensis is derived from the Latin words luna meaning "moon", and the suffix ~ensis, meaning "inhabitant". The specific 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 Sereno, Catherine Forster, Raymond R. Rogers, and Alfredo M. Monetta in 1993.
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. 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.
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.
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 the mouths of later theropods. The unique heterodont dentition of Eoraptor (Sereno et al., 1993) consists of both serrated, recurved teeth in the upper jaw, like the teeth of later theropods, and leaf-shaped teeth in the lower jaw, like the teeth of basal sauropodomorphs. Eoraptor had 4 teeth in the lower jaw and 18 teeth in the upper jaw, a dental formula not dissimilar to that of Herrerasaurus.
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.
- Alcober, Oscar A.; Martinez, Ricardo N. (2010). "A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina". ZooKeys. 63 (63): 55–81. doi:10.3897/zookeys.63.550. PMC . PMID 21594020. 
- Paul C. Sereno, Ricardo N. Martínez & Oscar A. Alcober (2013) Osteology of Eoraptor 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
- 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. ISBN 0-520-24209-2.
- Tykoski, 2005. Anatomy, ontogeny and phylogeny of coelophysoid theropods. PhD Dissertation. University of Texas at Austin. 553 pp.
- Sereno, P.C., Forster, C.A., Rogers, R.R., and Moneta, A.M. (1993). Primitive dinosaur skeleton from Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
- Langer, M. C., and Benton, M. J., 2006, Early Dinosaurs: a phylogenetic study: Journal of Systematic Palaeontology, v. 4, n. 4, p. 309-358.
- Liddell, Henry George and Robert Scott (1980). A Greek-English Lexicon (Abridged Edition). United Kingdom: Oxford University Press. ISBN 0-19-910207-4.
- Paul G.S., The Princeton Field Guide to Dinosaurs (Princeton University Press, 2010), p. 68.
- P. C. Sereno. 1995. Theropoda: early evolution and major patterns of diversification. Journal of Vertebrate Paleontology 15(3, suppl.):52A-53A
- Nesbitt, S. J.; Smith, N. D.; Irmis, R. B.; Turner, A. H.; Downs, A.; Norell, M. A. (2009). "A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs". Science. 326 (5959): 1530–1533. Bibcode:2009Sci...326.1530N. doi:10.1126/science.1180350. PMID 20007898.
- Bergman D.S., Sues H-D. (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.
- 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
- 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. In Encyclopedia of Dinosaurs (P.J. Currie, and K. Padian, Eds.) pp 731-736. Academic Press, San Diego, California.
- Ricardo N. Martinez, Paul C. Sereno, Oscar A. Alcober, Carina E. Colombi, Paul R. Renne, Isabel P. Montañez, Brian S. Currie. A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea. Science, Vol. 331, January 14, 2011, p. 206.
- Kaplan M, "Move over Eoraptor", http://www.nature.com/news, 13-1-2011.
- Apaldetti, C; Martinez, RN; Alcober, OA; Pol, D (2011). "A New Basal Sauropodomorph (Dinosauria: Saurischia) from Quebrada del Barro Formation (Marayes-El Carrizal Basin), Northwestern Argentina". PLoS ONE. 6 (11): e26964. doi:10.1371/journal.pone.
- Baron, M.G., Norman, D.B., and Barrett, P.M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543: 501–506. doi:10.1038/nature21700
- Tucker, Maurice E.; Benton, Michael J. (1982). "Triassic environments, climates, and reptile evolution" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 40 (4): 361–379. doi:10.1016/0031-0182(82)90034-7. Retrieved 2009-07-23.
- Columbi, Carina E. (2008-10-05). Stable isotope analysis of fossil plants from the Upper Triassic Ischigualasto Formation in the northwest of Argentina. Houston, TX: The Geological Society of America. Retrieved 2009-07-23.
- Sereno, P.C.; Novas, F.E. (1992). "The complete skull and skeleton of an early dinosaur". Science. 258 (5085): 1137–1140. Bibcode:1992Sci...258.1137S. doi:10.1126/science.258.5085.1137. PMID 17789086.
- Rogers, R. R.; Swisher III, C.C.; Sereno, P.C.; Monetta, A.M.; Forster, C.A.; and Martinez, R.N. (1993). "The Ischigualasto tetrapod assemblage (Late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins". Science 260 (5109): 794–797.
- Bonaparte, J.F. (1970). "Annotated list of the South American Triassic tetrapods". Gondwana Symposium Proceedings and Papers. 2: 665–682.
- Rogers, R. R., Swisher, C. C. III, Sereno, P. C., Monetta, A. M., Forster, C. A., and Martinez, R. N., 1993, The Ischigualasto Tetrapod Assemblage (Late Triassic, Argentina) and 40Ar/39Ar Dating of Dinosaur Origins: Science, v. 260, p. 794-797.