Eoraptor

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha

Overview: In the early 1990’s, a set of fossils belonging to a small bipedal dinosaur were unearthed in the San Juan Province of Argentina. The discovery was immediately recognized as significant, as the bones came from the famous Ischigualasto Formation, which dates to over two hundred and thirty million years ago, in the Late Triassic, which would automatically make the specimen one of the oldest ever found for a dinosaur. This would be reflected in the animal’s generic name, coined in its 1993 description – Eoraptor, the “dawn thief”. Despite what its name might imply, Eoraptor is not classified close to the “raptor” family Dromaeosauridae. The utilized root word is often used for smaller, presumably agile theropod dinosaurs. Those who described Eoraptor originally saw it as one of the most basal theropod taxa yet discovered, but later studies would challenge this idea. Nowadays, Eoraptor is usually associated with or classified within the Sauropodomorpha, sharing a closer common ancestor with dinosaurs like Plateosaurus or Diplodocus, than with Tyrannosaurus or Velociraptor. Even as such, it was one of the most basal of the sauropodomorphs.

Superficially, there were many theropod-like traits about Eoraptor, but this is likely due to it being closer to the common ancestor of sauropodomorphs and theropods. It would’ve retained a lot of traits from said ancestor that were lost in more derived sauropodomorphs. For instance, its neck, while slightly elongated, wasn’t nearly to the extent of those of its later relatives. The skull, while small, was proportionately larger too. The proposed sauropodomorph-theropod ancestor was, in all likelihood, a carnivore. Based on its teeth, which were differently-shaped throughout its mouth, Eoraptor was probably omnivorous, with herbivory evolving later in the Sauropodomorpha. All of the earliest sauropodomorphs, like Eoraptor, were strictly bipedal. Eoraptor, similar to other early dinosaurs, was quite small. The carnivorous Herrerasaurus is also known from the Ischigualasto and grew to be decently large for its time, but it was far from its region’s apex predator. That title went to the large non-dinosaurian, crocodile-line archosaurs like Saurosuchus, which could grow to be six or seven meters long and were powerfully built terrestrial predators.