Eoraptor is one of the earliest known dinosaur genuses, from approximately 231-228 million years ago, in the Late Triassic Period. It belonged to the basal sauropodomorphs and lived in those parts of western Gondwana which form northwestern Argentina now. Its type species, Eoraptor lunensis, was described in 1993 on the basis of an almost complete, well preserved skeleton, and we also know several fragmentary skeletons, which were useful in elucidating characteristics that were less exposed in the holotype specimen. The meaning of its name is 'dawn plunderer from the Valley of the Moon'.
Eoraptor lunensis. By w:en:user:Debivort - From WP-en The colours of this image were altered., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1537356
Known occurrences of Eorpator fossils.
The fossil remains of Eoraptor were first discovered by Ricardo Martínez during a jointed field work of the University of San Juan and the University of Chicago in the Valle de la Luna ('Valley of the Moon') of the Ischigualasto Formation in Argentina, in muddy stiltstone of the Cancha de Bochas - the formation and its fossils were dated approximately to 235 to 228 million years ago, to the Carnian stage of Late Triassic. The fossil was first displayed in the Field Museum of Natural History in Chicago, where it has been preparated, and then it was transported back to Argentina, where it is currently displayed in the Museum of Natural Sciences, San Juan. Eoraptor was scientifically described by Paul Sereno and his co-authors, Catherine Forster, Raymond R. Rogers, and Alfredo M. Monetta in 1993.
Skeleton of Eoraptor lunensis. By The Lord of the Allosaurs - File:Royal Ontario Museum Eoraptor.JPG, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=39634464
Skull of Eoraptor lunensis. By Tylwyth Eldar - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=93326062
Classification and valid species
The only known species of the genus is Eoraptor lunensis, described in 1993 from northwestern Argentina. As this is one of the earliest dinosaurs, and probably lived before the major dinosaur groups developed most of their important characteristics, it was not an easy task to classify Eoraptor. However, the fact that unlike many basal saurischians, in case of Eoraptorthe complete skull and postcranium are known, was definitely helpful during this scientific task. It was originally regarded by Paul Sereno for an early theropod dinosaur. Nevertheless, its precise classification was unstable, and it often was treated as either a basal saurischian or a basal theropod. The most recent comprehensive studies regard Eoraptor for an early sauropodomorph and not for a theropod.
The discovery of Eoraptor and other basal dinosauria, like the more advanced theropod Herrerasaurus and the ornithischian Pisanosaurus, helped a lot in our understanding on the early evolution of Dinosauria. It seems that during the early Carnian, dinosaurs diverged rapidly from a common ancestor, and in this time most of their species had a small body size, and the divergence between the principal herbivorous and carnivorous lineages was established by the middle Carnian.
Eoraptor lunensis. By Conty - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=16984546
Eoraptor was a lightly built and slender dinosaur with a body length of maximum 1 meters and weighed about 10 kilograms. The skull was elongated and its naris was slightly enlarged, furthermore, there was a kink in its upper jaw. The premaxillary and anterior maxillary teeth of Eoraptor were leaf-shaped, and its premaxilla had a slender posterolateral process .
Eoraptor had double as long hindlegs than its forelimbs, which allows to conslude that it might be a bipedal dinosaur. Furtherormore, the bone structure of its legs suggests that it was a digitigrade animal, meaning that it walked on its toes and did not touched the ground with its heels while walking. Probably it was a quick dinosaur and also a swift runner. Eoraptor had five digits on each of its limb, with large claws in the three longest on each feet; these could be used for handling the prey. Eoraptor kept the characteristic of some of its ancestors in having hollow vertebral centra. Its ilium was supported by three sacral vertebrae, while normally it was supported by two in basal sauropodomorphs. In case of Eoraptor, unlike in most saurischians, there were no pneumatic cavities in the presacral vertebrae; however, the long bones of the skeleton were strongly hollowed, which is a theropod characteristic.
In its original description Paul Sereno et al. suggest that the type specimen was an adult, according to its closure of sutures in the vertebral column, furthermore, the partial fusion of the scapulocoracoid also supports this view. However, other researchers suggest that the type specimen was a young animal, as it possessed a relatively large orbital opening in the skull, a short snout, and the fact that the skull bones are not completely fused. Max Langer and Michael Benton observed that the proximal part of fibula was extremely transversely compressed in case of this dinosaur.
Human - Eoraptor size comparison. By Nobu Tamura email:email@example.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/ - http://spinops.blogspot.dk/2015/07/eoraptor-lunensis.html?q=Eoraptor, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50251427
Paleobiology and paleoecology
Eoraptor is considered to have been an omnivorous dinosaur, which means that it consumed a variety of food of both plant and animal origin. Its fast running ability made possible for Eoraptor to chase its prey, and when Eoraptor caught it, probably used its teeth and claws to bring down and tear apart its victim.
Eoraptor differed from modern carnivorous dinosaurs in several traits - at the articulation of the lower jaw, it did not possessed a sliding joint, therefore it was unlikely that Eoraptor was able to hold larger prey with its jaws; in addition, Eoraptor had only a few curved and serrated teeth, which were characteristic for modern theropods. Insted, it had a heterodont dentition - it had some leaf-shaped teeth in the lower jaw (characteristic for basal sauropodomorph), and also recurved and saw-edged teeth in the upper jaw (characteristic for theropods) . It is important to mention, that the dental formula of Eoraptor (4 teeth in the premaxilla and 18 in the maxilla) was different than that of Herrerasaurus, an other early dinosaur with key importance in understanding the evolutionary history of dinosaurs.
The habitat of this dinosaur, the Ischigualasto Formation, was a forest-covered foodplain with volcanic activity during the Late Triassic, probably with a warm and humid clomate. This area probably also had a seasonal variation. Based on fossil evidence, vegetation of these forests consisted of moss, horsetails, ferns and giant conifers. Eoraptor probably shared on its environment with a few other dinosaurs like Herrerasaurus and Pisanosaurus, and in addition, with archosaurs, therapsids, carnivorous cynodonts and rauisuchians. The herbivore fauna at that time consisted mostly of non-dinosaur groups, like rhynchosaurs, cynodonts and traversodontids. Based on the yet explored fossils, dinosaurs represented only about 6% of the fauna of the Ischigualasto Formation during the Late Triassic Period.
Digital drawing of the known skeletal remains of Eoraptor lunensis. Known elements represented in white and unknown in gray. By Maurissauro - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=91236381
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