Manidens, an early ornithischian by Julio Lacerda
Ornithischians all have beaks, distinctive cheek teeth, and bird-like hips. Meanwhile, theropods and sauropodomorphs are thought to be close relatives, in a group called Saurischia. Saurischians possess a unique articulation between the neural spines of their vertebrae, as well as an assymetrical hand where the second digit is the longest rather than the third.
Paleontologists Matthew Baron, David Norman, and Paul Barrett have published an unexpected family tree of the dinosaurs based on an analysis including most early dinosaurs and their relatives compared in more than 450 different anatomical features. They found that the family tree of dinosaurs is significantly different than previously thought, with the sauropodomorphs branching off from the others first. Ornithischians are surprisingly most closely related to the theropods, forming a group they call Ornithoscelida.
Sefapanosaurus, an early sauropodomorph by Chris Masna
Early ornithoscelidans are linked by several anatomical features previously considered to be the result of convergent evolution. They include a gap in the upper tooth row known as a diastema, a sharp bony edge on the lower margin of a large facial sinus called the antorbital fenestra, and several features of the braincase. In the rest of the body, orithischians and theropods share a very long narrow scapula, a curved humerus, similar muscle attachment points on the femur, and several small ankle bones fused to the bones of the feet. In short, they find the evidence supporting a group containing ornithischians and theropods to be greater than the evidence supporting a group containing theropods and sauropodomorphs.
Staurikosaurus, a herrerasaurid by Julio Lacerda
The researchers also found that the herrerasaurids, an early group of carnivores usually considered to be theropods, are actually most closely related to the sauropodomorphs. The herrerasaurids and sauropodomorphs both share an oval antorbital fenestra and several features of the hips and hindlimbs. They retain the name Saurischia for the group including herrerasaurids and sauropodomorphs.
Baron and colleagues acknowledge that this arrangement is novel and has never before been proposed by paleontologists, but may answer some questions about what the earliest dinosaurs looked like. They propose that the earliest dinosaurs were actually omnivorous bipeds, and that the two carnivorous lineages, the herrerasaurids and theropods, evolved carnivorous lifestyles independently. Additionally, they find that the first dinosaurs evolved in the northern portion of the ancient supercontinent of Pangea, a conclusion at odds with two decades of research suggesting the first dinosaurs evolved in southern Pangea.
They also propose that feathers, and the feather-like skin structures seen in numerous theropods and some ornithischians share a single evolutionary origin, and are limited to the ornithoscelidans among dinosaurs. This would mean that sauropodomorphs and herrerasaurids never possessed feathers.
Tawa, an early theropod by Nathan E. Rogers
The ideas proposed by Baron and colleagues are some of the more unique proposals for the evolutionary history of dinosaurs, and are sure to lead to heated discussion among paleontologists. Further research as well as more fossils of early dinosaurs will surely support or falsify the claims made by these authors.