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

The earliest fully aquatic cetaceans (Middle to Late Eocene), had a serpentine body shape which was adapted for locomotion by axial undulation. This has drawn comparisons with the mosasaurs, a group of Late Cretaceous marine squamates, with whom these early cetaceans also share broad similarities in skull morphology, including a triangular snout with widely spaced teeth and an unfused mandibular symphysis. Both groups filled a wide range of niches and achieved global distributions. As they became increasingly adapted to aquatic life, each clade modified their body plan to allow more efficient tail-based ‘thunniform’ locomotion, which enabled colonisation of the open ocean. Cetaceans continued to diversify after reaching this form whereas the evolutionary history of the mosasaurs was cut short by the end-Cretaceous mass extinction.
We investigated the evolution of skull ecomorphology through the initial aquatic radiations of mosasaurs and cetaceans. Thirteen ecologically informative craniodental measurements and ratios were taken from 3D models of well-preserved skulls. The initial dataset consisted of ten cetaceans and eleven mosasaurs, sampling across the first twenty million years of evolution in each group. These data were subjected to ordination techniques in R to reconstruct patterns of morphospace occupation. Preliminary results show that Eocene cetaceans had a conserved skull morphology, only branching out into new morphologies in the Oligocene (early odontocetes significantly more so than early toothed mysticetes). By comparison, mosasaurs show much more ecomorphological variation. Future work will use 3D landmarks to accurately compare differences in skull shape through the evolution of each group.