Eoraptor is one of the earliest dinosaurs discovered, from 215 mya in the Late Triassic. Until recent discoveries came to light, it was thought to be the earliest theropod. However, new studies show that it was one of the first ancestors of the mighty sauropods. However, Eoraptor was barely bigger than a dog. You would never think that it was related a giant like Brontosaurus.
Eoraptor would have eaten insects, small reptiles, and anything else it could get its hands on. When it was discovered, people thought it was a theropod because of the slender, bird-like head and sharp teeth that lined the jaws, but the family tree of dinosaurs has been turned upside-down, and scientists decided that Eoraptor was a sauropodomorph, one of the ancestors of the largest animals ever.
Eoraptor lived in Argentina along with many (mostly carnivorous) dinosaurs, such as the Herrerasaurids, which were almost nearly confined to South America. Herrerasaurus and Staurikosaurus were one of the largest dinosaurs during this time. The Herrerasaurids, although they had theropod-like features, are now believed to be their own group related to sauropodomorphs, with ornithischians and theropods on the other side of the family tree.
Eoraptor was thought to have been the earliest dinosaur, but this is not proven. The earlier remains of an animal called Nyasasaurus from Tanzania have been dated to 245 mya, and may be from the first dinosaur. It is at least the closest thing to a dinosaur. For now, Eoraptor is the earliest definitive dinosaur. However, science is ever-changing and the next thing we may know is that Eoraptor was not even a dinosaur!
Anay Ganesh 