【化石】恐竜の系統樹を大きく揺さぶる新しい仮説

A new hypothesis about the history of the evolutionary relationships of dinosaurs that challenges over a century of dogma is presented in this week’s Nature. The theory proposes a radical regrouping into two new major dinosaur groups. If confirmed, the dinosaur family tree may require a re-write.

For nearly 130 years, dinosaurs have been divided into two major groups, or clades: the Ornithschia - characterized by a ‘bird-hipped’ pelvis, and the Saurischia, which have a reptile-like pelvis. The Ornithschia includes the ornithopods such as Iguanodon and armoured dinosaurs such as Triceratops and Stegosaurus; the Saurischia includes carnivorous theropods such as Tyrannosaurus rex, and the giant sauropods such as Diplodocus.

Matthew Baron and colleagues examine a wide range of early dinosaurs (74 taxa), analysing 457 characteristics to test for anatomical similarities and differences. They recover a new clade in which Ornithschia and theropods are grouped together, supported by 21 characteristics inherited from a common ancestor, such as sharp ridges on the jawbones and distinctive metatarsals, and a vast array of other shared features. The second regrouping places sauropods with early carnivores known as herrerasaurs, rather than carnivorous theropods (the authors suggest that similar features seen in these meat-eaters may have evolved independently).

The model also provides new clues about the evolution of dinosaurs. For example, it suggests that early dinosaurs were omnivorous, small, walked on two legs, and had grasping hands. Moreover, the analysis indicates that dinosaurs may have originated in the Northern Hemisphere, rather than in Gondwana, although the authors propose that the timing and geographical setting of dinosaur evolution may require reappraisal.