A New Theory of Dinosaur Evolution

Say Hello to a Proposed New Dinosaur Family, the "Ornithoscelidae"

Eoraptor, the incumbent candidate for "first dinosaur". Wikimedia Commons

It's not often that a scholarly paper about dinosaur evolution shakes up the world of paleontology and is covered in major publications like The Atlantic and The New York Times. But that is exactly what has happened with a paper published in the British magazine Nature, "A New Hypothesis of Dinosaur Relationships and Early Dinosaur Evolution," by Matthew Baron, David Norman and Paul Barrett, on March 22, 2017.

What makes this paper so revolutionary? To grasp this requires a quick briefing on the currently existing, widely accepted theory about the origin and evolution of dinosaurs. According to this scenario, the first dinosaurs evolved from archosaurs about 230 million years ago, during the late Triassic period, in the part of the supercontinent Pangea that corresponds to modern-day South America. These first, small, relatively undifferentiated reptiles then split off into two groups over the next few million years: saurischian, or "lizard-hipped," dinosaurs, and ornithischian, or "bird-hipped," dinosaurs. Saurischians include both plant-eating sauropods and meat-eating theropods, while ornithischians comprise everything else (stegosaurs, ankylosaurs, hadrosaurs, etc.).

The new study, based on a lengthy, detailed analysis of dozens of dinosaur fossils, presents a different scenario. According to the authors, the ultimate ancestor of dinosaurs originated not in South America, but in the part of Pangea roughly corresponding to modern-day Scotland (one proposed candidate is the obscure, cat-sized  Saltopus). The first "true" dinosaur, moreover, is proposed to be Nyasasaurus, which originated in the part of Pangea corresponding to modern-day Africa--and which lived 247 million years ago, ten million years earlier than previously identified "first dinosaurs" like Eoraptor.

More importantly, the study completely rearranges the lowest branches of the dinosaur family tree. In this account, dinosaurs are no longer divided into saurischians and ornithischians; rather, the authors propose a group called Ornithoscelidae (which lumps in theropods along with ornithischians) and a redefined Saurischia (which now includes sauropods and the family of meat-eating dinosaurs called herrerasaurs, after the early South American dinosaur Herrerasaurus). Presumably, this classification helps account for the fact that many ornithischian dinosaurs possessed theropod-like characteristics (bipedal postures, grasping hands, and in some species, even feathers), but its further implications are still being worked out.

How important is all this for the average dinosaur enthusiast? Despite all the hype, not very. The fact is that the authors are looking back to a very opaque time in dinosaur history, when the earliest branches of the dinosaur family tree had yet to be established, and when it would have been virtually impossible for an observer on the ground to distinguish between a profusion of two-legged archosaurs, two-legged theropods, and two-legged ornithischians. Turn the clock ahead tens of millions of years to the Jurassic and Cretaceous periods, and everything pretty much remains unchanged--Tyrannosaurus Rex is still a theropod, Diplodocus is still a sauropod, all is right with the world.

How have other paleontologists reacted to the publication of this paper? There is widespread agreement that the authors have done careful, detailed work, and that their conclusions deserve to be taken seriously. However, there are still some objections being voiced about the quality of the fossil evidence, especially as it pertains to the earliest dinosaurs, and most scientists agree that additional, confirming evidence will be needed before books on dinosaur evolution have to be rewritten. In any case, it will take years for this research to filter out to the general public, so there's no need to worry just yet about how to pronounced "ornithoscelidae."