The Dawn Runner and the Dethroning of Eoraptor

The origins of dinosaurs have always been a little bit mysterious–they seem to spring onto the Triassic stage fully formed, carnivores and herbivores, all of them recognizably dinosaurian. The transitional phases, so important to evolutionary paleontology, seem to be missing. But a recent paper by perennial favorites Paul Sereno and Ricardo Martinez has both done an interesting bit of taxonomic shuffling and named a new early dinosaur.

Eodromaeus murphii is a small predator from the almost ludicrously productive Ischigualasto Formation, which holds rock about 231 million years old. Like its contemporary (and better known) relative Herrerasaurus, Eodromaeus is certainly a theropod. It has all the hallmarks; a long, low skull filled with sharp, recurved teeth and the the long, lanky build common to early predatory dinosaurs. It comes from a time when the various families of dinosaurs were beginning to split off into the families we would come to recognize; theropod relatives evolving into the massive sauropods, the herbivorous ornithopods beginning a massive adaptive radiation that would see them become everything from duck-billed hadrosaurs to the horned ceratopsians.

However, it’s rather unclear how many of these transitions took place. The recently discovered Panphagia is an example of where the theropod/sauropod devide may have been, but it still left some unanswered questions concerning how exactly theropods got to the point of exchanging their sharp pointy teeth for ones better suited to devouring greens.

It’s the teeth, actually, that lead to the other, more startling assertion. The previous holder of the title for earliest theropod was a small predator known as Eoraptor, also discovered by Mr. Sereno. According to the hypothesis put forward in the paper, though, Eoraptor is actually a transitional phase between theropod dinosaurs and the massive long necked sauropods–much like Panphagia. The authors point to its teeth–which are rather more leaf shaped then its contemporary killing cousins–as an excuse to place it more on the sauropod side of the taxonomic barrier. This means that it took a while for the two families to properly split, and that the shift to herbivory was not as clean cut as we may have thought.

Whether this sticks will remain to be seen, but at present its an interesting new twist on the continued search for the origins of dinosaurs.

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