Evolution of the axial skeleton of early dinosaurs and the role of meristic and homeotic processes

Changes in the axial skeleton played an important role in dinosaur evolution, with some of the oldest dinosaurs showing evidence of sacralization events. The increased number of early dinosaur discoveries over the last decade provides an opportunity to better characterize this phenomenon. It has led us to revise our understanding of axial evolution in early dinosaurs. Results support that the last common ancestor of Dinosauria would probably present 9 cervical, 16 dorsal and 2 sacral vertebrae (primordials). Early saurischians maintain a consistent number of vertebrae with presacral origin, and there is no evidence of meristic changes in these dinosaurs. A more complex scenario is observed in early ornithischians (including ‘silesaurids’). An insertion between the primordials occurs early in ornithischian evolution, and it appears to be maintained in ‘traditional’ ornithischians (Saphornithischia). Unambiguous evidence supports loss and gain of presacral vertebrae in ornithischians. Inserted sacrals across all dinosaurs are associated with a loss of a corresponding dorsal vertebra, supporting that these represent trunk vertebrae moved into the sacrum rather than novel vertebral unities. Changes in the axial skeleton of early dinosaurs are dominated by homeotic processes, with meristic processes restricted to derived members.