Scientific Classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Eusaurischia
Genus: Eoraptor

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The Eoraptor is considered one the first dinosaurs to exist on the earth. It was a South American dinosaur that was present in the mid Triassic period. It was in existence 232 to 225 million years ago.
The Eoraptor was very small dinosaur, possibly weighing no more than 8 to 12 kilos. It prominently shows carnivorous traits in its jaw bones, but these are not as well defined as those of the more evolved carnivorous dinosaurs. This indicates that the Eoraptor could have been an omnivore.
It shows features similar to both theropods and sauropods. It is speculated that both these sub orders could have evolved from the Eoraptor. It is currently classified under order Saurischia.
The fossils of the Eoraptor were discovered in Argentina.

Nomenclature
The word 'Eos' is used to denoted the goddess of dawn in Greek mythology. The word 'raptor' translates to 'destroyer' or 'plunderer' in Latin.
The Eoraptor is one of the earliest known dinosaur and hence the epithet 'Eos' is used in its name. It was believed to tear apart its prey with its developed forelimbs and hence the suffix 'raptor' is used for the Eoraptor.
The species name of the organism is Eoraptor lunensis. The fossils of the Eoraptor were found in a region called 'Valle de la Luna' which means 'valley of the moon' in Spanish. The suffix '-ensis' roughly translates to 'being from'. Thus ‘lunensis’ is supposed to denote 'from the moon', taking into account the location of the discovery of fossils.
The name was suggested by scientists Sereno, Forster, Rogers and Monetta.

Taxonomic Classification

• The exact position of the Eoraptor within clade Dinosauria has been a matter of debate. When the remains were first found in 1991, it was believed to a primitive theropod. A group of scientists led by Paul Sereno classified it as a theropod in 1993 due to the three functional digits with claws in its fore legs. Theropods were known to have grasping features.
• Currie determined in 1997 that the Eoraptor resembled the predecessors of both sauropods and theropods, questioning the theropod heritage ascribed to it.
• Sues and colleagues determined in 2011 that the Eoraptor was indeed a basal theropod based on other theropods being discovered, namely Tawa and Daemonosaurus. They deduced that the Eoraptor more closely resembled these dinosaurs than sauropods.
• A group of scientists led by Martinez published a paper in 2011 stating that the Eoraptor was more closely related basal sauropodomorphs. A little later in the same year, Apaldetti and colleagues recorded that they could classify the Eoraptor only as a saurischian. They found it related to both sauropods and theropods. They refrained from classifying the remains further.
• Sereno again modified the classification of the Eoraptor, stating it to be more closely related to basal sauropodomorphs.

The Eoraptor should not be mistaken to be related to the more popular Velociraptor.

Discovery of fossils
The fossils of the Eoraptor were found in the Ischigualasto Formation in Argentina. It was discovered by paleontologist Ricardo Martinez of the University of San Juan. This endeavor was undertaken in collaboration with the University of Chicago. The remains were found in the Valle de la Luna region or the Ischigualasto Provincial Park. The fossils were first shipped to Chicago to be displayed in the Field Museum of Natural History. They are currently housed and displayed at the Museum of Natural Sciences in Argentina.
These fossils are presumed to be those of the earliest dinosaurs. The Staurikosaurus is considered more ancient than the Eoraptor, but its herbivorous habits have made scientists doubt the premise.

The Ischigualasto Formation
The Ischigualasto Formation is a part of the Ishigualasto Provincial Park in Argentina.

• The rock formation of the Talampaya National Park overlaps at places with the Ischigualasto Provincial Park. Hence the formation is sometimes referred to as the Ischigualasto-Talampaya formation. It ranks even further than the Chinle Formation of North America in terms of the numbers and quality of the fossils discovered here.
• The structure currently consists of arid plains with very scanty wooded areas or water reserves. But in the Triassic period, it was abundant with lakes and swamps. Flooding in this region was very common in that period.
• The Ischigualasto Formation comprises predominantly of red sandstone. Grey clay-stone is also seen infrequently in the region.
• The fossils in this region are chiefly from the Carnian age of the Triassic period. Along with early dinosaurs, fossils of plants as well as other primitive reptiles and aquatic organisms have been found here.

The carnivorous Herrerasaurus is one of the most prominent discoveries of the Ischigualasto formation. It is believed that omnivorous dinosaurs such as the Eoraptor may have evolved into carnivorous species.

Nature of fossils
The Eoraptor was found is almost perfect condition in Argentina. Bones of the skull, forelimbs, spine, pelvis as well as those of the hind legs were discovered.

• The upper and lower jaw of the creature did not glide as easily as other carnivorous dinosaurs. But its teeth were pointed and sharp. There is evidence that the teeth were both curved and serrated, a feature which was not seen heretofore.
• Bones for five digits of the forelimbs were discovered. The tibia of the Eoraptor was found to be longer than the femur.
• The bones of the skull were initially thought to belong to a juvenile specimen but study of the vertebrae showed that it was not so.

Physical characteristics
The Eoraptor was a tiny dinosaur, growing to about 1 meter in length and about 10 kilos in weight. It stood upright and was unable to use all four limbs for walking.
It had a small head and a whip like tail. It had five digits on its forelegs, three of which had claws.
The long tibial bone indicates that the Eoraptor was a strong sprinter. The muscle attachments on its long-bones indicated that it was a lean creature.

Habits and habitat
The unique nature of the Eoraptor's teeth indicates that it was possibly an omnivore. The serrated teeth were adapted for tearing flesh and the rounded teeth were most likely present for chewing vegetation.
The claws in the forelimbs were most likely used to rip apart its prey. These features indicate carnivorous habits. Thus, either the Eoraptor was an omnivore or had omnivore ancestors.
The habitat of the Eoraptor comprised of rivers and wet lands. It most likely preyed on smaller aquatic organisms.

Related species
The Eoraptor is considered to be closely related to archosaurs. So scientists speculate it could itself be an archosaur. It possibly was related
to the other South American dinosaurs, the Herrerasaurus and Pisanosaurus. Its features also resembled therapids, rauisuchians and the Saurosuchus.

Final notes
The discovery of the Eoraptor was monumental in understanding the origin of dinosaurs. It has features resembling theropods as well as sauropods indicating that the two orders could possibly have evolved from dinosaurs such as the Eoraptor.
The carnivorous theropods such Herrerasaurus could have evolved from or co-existed with the Eoraptor.
Although the lineage of the Eoraptor is still debatable, its discovery has greatly enlightened the understanding of the evolution of dinosaurs in the Triassic period.