Digging in Argentina

Several skeletons of Eoraptor have been discovered since 1991 allowing quite thorough study of this animal. During the 1991 dig season the University of Chicago and the University of San Juan teams were busy combing the rocks of the Ischigualasto Formation when Ricardo Martinez happened to unearth bones which would lead to the discovery of the first Eoraptor skeleton.After two years of study and fossil finding the paper released with Sereno as head author announced the Eoraptor and described what they had found (Martinez was not a named author in the paper). A very basal dinosaur lacking many traits of its descendants, Eoraptor was thought at the time to be a carnivorous dinosaur. However, only about half of its teeth were curved and serrated and its mandible did not have a sliding articulation which would allow for the grasping of large prey like later carnivores. It did have three digit hands and feet like many later predators and the tridactyl hand was a large factor in the placement of the animal in the theropod lineage.

Since that time Sereno has released further research that places Eoraptor in the sauropodomorph, others have argued it is still a theropod and Martinez has led research which came to the conclusion that there was no exact argument for either branch which would safely put it in either one. The research considered Eoraptor an eusaurischian, an animal that is so basal it is neither theropod nor sauropodomorph but at the base of both family trees. This is a unique position as it tells us a lot about the structure of the earliest dinosaurs and the building blocks of later evolution and how structures form. There is still a lot to learn from the Triassic fossils of the Ischigualasto, but we have a good start thanks to the discoveries made in the Ischigualasto formation like Eoraptor and Herrerasaurus, amongst many others.