Eoraptor

*Eoraptor lunensis*

***Eoraptor*** (meaning "dawn thief") is a genus of sauropodomorph dinosaurs. It is widely considered to be one of the worlds earliest dinosaurs. The *Eoraptor* lived during the late Triassic Period, around 231 to 228 million years ago (mya), and is believed to resemble the ancestor of all dinosaurs. It is known by well-preserved fossil records found in Argentina.

*Eoraptor* had two different types of teeth, indicating that it ate both small animals and plants.

## Description

The *Eoraptor*, was given the name raptor but it has no relations to the *Velociraptor* or any dromaeosaurids. Like many early dinosaurs, had a relatively small body in comparison to dinosaurs of later periods. It had a thin body that was approximately 1 meter in length when fully developed, and weighed approximately 10 kilograms. A bipedal carnivore, the *Eoraptor* ran upright on its hind legs, while its forelimbs were only half the length of the hind legs. Each forelimb ended in a five digit hand, with three of the digits ending in large claws that were believed to be used to catch and handle prey. While it was believed to have eaten small animals, due to its like swift speed and sharp claws, the *Eoraptor* also possessed both herbivorous and carnivorous teeth, leading to suggestions that Eoraptor may have been omnivorous.

## Discovery

The fossil remains of the *Eoraptor* were first discovered in 1991 by paleontologist Paul Sereno, in the Ischigualasto Basin located in Argentina. Although the area is now barren as a desert badlands, it was believed to have been a river valley during the late Triassic. The *Eorapto*r was found in the Ischigualasto Formation, which was also the same foundation in which the early theropod *Herrerasaurus* was located. As the *Eoraptor* lacked the specialized hunting features of the major groups of later carnivorous dinosaurs, such as a sliding joint within the articulation of the lower jaw, which would have been used to hold larger prey, it was determined to be one of the world's earliest dinosaurs in 1993. Further evidence supporting the claim of its age was the fact that only some of the *Eoraptor's* teeth were curved and serrated, unlike later predators.

### Classification

As the *Eoraptor* possessed both herbivorous and carnivorous teeth, as well as five fully developed fingers, it was suggested that the *Eoraptor* was a more ancient species than even the *Herrerasaurus*. It is believed that only some prosauropods recently discovered in Madagascar of older age. The *Staurikosaurus* is considered to be possibly older, but due to its large size, it is expected to be of a later evolutionary period. However, as *Staurikosaurus* seems to have qualities present in both prosauropods and theropods, similar to *Eoraptor's* shared herbivorous and carnivorous tendencies, it has led researchers to question *Eoraptor's* primitive label, as its suggested omnivorous qualities were a key factor in its age deduction.

## 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.

## JPInstitute.com Description

Eoraptor is one of the earliest dinosaurs ever discovered. That means that this small dinosaur was one of the first to ever walk on Earth, and is a basal sauropodomorph, and it helped determine what other dinosaurs would look like. Although it was small, it was a fierce predator. Its speed and intelligence are some of the reasons that dinosaurs replaced other animals as the dominant land creatures.

Discovered in 1993 in the Ischigualasto Basin in northwestern Argentina, the small skull was found in a single rock. Unlike most dinosaur discoveries, and to the delight of the team that found this important creature (Paul Sereno, Fernando Novas, and their team), an almost complete skeleton was found. This single creature has greatly increased scientists' knowledge of how dinosaurs developed and evolved. There are so few dinosaurs known from this time period that finding a complete skeleton of such an early member of the dinosaur family is a big help in expanding our knowledge of the early dinosaurs.

Eoraptor had the characteristics of later dinosaurs - serrated teeth, grasping hands (although there is some speculation that it occasionally walked using all four limbs), light hollow bones and a strong, light skull. Although it lived at the same time as the larger Herrerasaurus (which may have eaten Eoraptor), it has some significant differences. Some of its teeth were shaped differently and the bones in its hands were more primitive. Later dinosaurs tended to lose fingers, and by the time T. rex came onto the scene, it had only two fingers. Eoraptor had five. Even Herrerasaurus, which had five fingers, had a less useful fifth finger.

## Dinosaur Field Guide Description

Eoraptor ("dawn hunter") is the most primitive known dinosaur, and for paleontologists it serves as a model for what the very first dinosaur probably looked like. Before Eoraptor was found, scientists looked at the oldest and most primitive members of each of the major groups of dinosaurs. They saw that most of these were only about 3.3 feet (1 m) long, and that they walked on their back legs. Then, in 1991, Ricardo Martinez found the skeleton of Eoraptor in Argentina. It matched what paleontologists had expected to find in a common ancestor. It was only 3.3 feet (I m) long, and it walked on its back legs. Was Eoraptor the ancestor of all dinosaurs? Unfortunately, no. It lived too late in time. Other, more advanced dinosaurs were found in the same Late Triassic rocks where Eoraptor was found. But paleontologists haven't given up the search for that very first dinosaur. They are now searching for new fossils in even older rocks.

### Fun Facts

Eoraptor was found in the Valley of the Moon.

### Trivia

When the thighbore of Eoraptor was first found, paleontologists at first thought it was a little crocodile relative. They realized that it was a dinosaur when the rest of the skeleton was dug up.

## Links

http://web.archive.org/web/20031006083631fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_eorap.html