Charting new waters: changes in skull ecomorphology during the initial aquatic radiations of mosasaurs and cetaceans

The repeated return of tetrapods to water provides many iconic examples of convergent evolution, with various groups of mammals and reptiles independently evolving streamlined body shapes and similar feeding strategies. One comparison which has received little attention is that of cetaceans (whales and dolphins) and mosasaurs (a group of Late Cretaceous marine squamates). The earliest fully aquatic members of both groups had serpentine bodies and swam by axial undulation, before evolving more efficient caudal oscillatory locomotion and colonizing open ocean niches. Here we investigate possible parallel evolutionary trajectories of skull morphology that occurred during these initial aquatic radiations. A series of functionally informative ratios were calculated from 32 species of mosasaur and early cetacean. These were subjected to ordination techniques to reconstruct patterns of functional morphospace occupation. Preliminary results show that the earliest mosasaurs had gracile skulls specialized for smaller prey, from which they radiated in several waves across the morphospace. By contrast, basilosaurid cetaceans occupied a relatively constrained megapredatory niche, and only evolved new ecomorphologies after the Late Eocene split into odontocetes and mysticetes. Theresults also suggest cranial convergence between the toothed mysticete Janjucetus and the mosasaur Prognathodon. Future work will investigate these results further using 3D landmarks.