Too early to rewrite the textbooks
The early evolution of the dinosaurs remains a matter of lively debate, and a recent attempt to reorganize their family tree has now elicited a robust riposte from a team of researchers that includes LMU paleontologist Oliver Rauhut.
A paper written by paleontologists from Cambridge University and the Natural History Museum in London, and published earlier this year in Nature, made headlines beyond the confines of science journals. This is hardly surprising, given the paper’s content – for its authors challenged the picture of dinosaur evolution that has been accepted for over a century. The traditional classification of the Dinosauria divides them into two major groups, based on the form of the pelvis: The “bird-hipped” ornithischians include the massive herbivores such as Stegosaurus, Triceratops or Iguanodon, while the lizard-hipped saurischians make up the second major group. The latter comprise the iconic bipedal carnivores (Theropoda) and the long-necked Sauropoda. In their 2017 paper in Nature, paleontologist Matthew Baron and his co-authors proposed a radical reconfiguration of this phylogeny, in which the theropods were grouped together with the saurischians in an overarching taxon called the Ornithoscelida. Now an international group of dinosaur experts, including LMU’s Oliver Rauhut (who is also affiliated with the Bavarian State Collections for Paleontology and Geology), has come to a very different conclusion, and they too set out their case in Nature.
The classification proposed by Baron and his colleagues places the carnivorous dinosaurs in the same group (“clade”) as the bird-hipped ornithiscians, leaving only the long-necked lizard-hipped forms in the newly defined Saurischia. This reorganization of the conventional phylogeny of the dinosaurs also implied that they originated not in the southern half of the supercontinent Pangea (as is generally believed) but to the north of the equator.
Given the radical nature of this proposal, Rauhut and his co-authors have critically reanalyzed the morphological dataset on which it is based, pointing out that its authors had actually been able to study only about half of the characters used in their investigation themselves (details of the other half were drawn directly from the relevant research literature). Led by Max Langer of the University of Sao Paulo in Brazil, members of the international consortium have now re-examined at first hand hundreds of dinosaur fossils from all over the world.
Based on this re-analysis, Rauhut and his colleagues conclude that the traditional division into Ornithischia and Saurischia depicts the most likely evolutionary scenario. Other possible patterns of relationships cannot be conclusively ruled out, but they are not as well supported by the evidence available as the traditional topology of the dinosaurs’ family tree. Since they all display very similar character sets the classification of the earliest ancestors of the three major groups is an enormously difficult task, as Rauhut readily admits. But he and his team have no doubts as to the geographical origin of the dinosaurs: all the evidence argues that they first evolved in southern Pangea, in other words, in the southern hemisphere. Rauhut: “It’s too early to rewrite the textbooks.”