Leviathans Unleashed: Skull Ecomorphological Evolution During The Initial Aquatic Radiations Of Mosasaurs And Cetaceans

The repeated return of tetrapods to aquatic environments 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 that has received little attention is that between mosasaurs (a group of Late Cretaceous marine squamates) and early cetaceans (middle to late Eocene ancestors of modern whales and dolphins). These two groups share broad similarities in skull morphology, filling a wide range of niches and achieving global distributions. The earliest fully aquatic members of both groups had serpentine bodies and swam by axial undulation, before evolving more efficient caudal oscillatory locomotion and colonising open ocean niches. Cetaceans continued to diversify after reaching this form whereas the evolutionary history of the mosasaurs was cut short by the end-Cretaceous mass extinction. 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 mosasaurs and early cetaceans. These were subjected to ordination techniques to reconstruct patterns of functional ecomorphospace occupation, and putative examples of convergence were tested statistically. Preliminary results show that the earliest mosasaurs had gracile skulls, specialised for smaller prey, from which they radiated in several waves across the ecomorphospace. There is considerable variation within certain genera, such as Mosasaurus. By contrast, basilosaurid cetaceans occupy a relatively constrained megapredatory niche and cetaceans only evolved new ecomorphologies after the late Eocene split into odontocetes and mysticetes. Oligocene odontocetes explore a new area of morphospace away from the basilosaurids, evolving a long, narrow snout with an increased number of small teeth. The earliest toothed mysticetes have a similar ecomorphology to the basilosaurids, with aetiocetids appearing to radiate in a similar direction to the odontocetes. The late Oligocene Janjucetus, which has a highly unusual ecomorphology, plots away from other cetaceans. Despite showing striking similarities to the mosasaur Prognathodon (e.g short robust snout and large eyes), the two taxa were not found to be statistically convergent. However, cranial convergence was found between the mosasaur Mosasaurus hoffmanni and the basilosaurid Dorudon atrox. Future work will investigate these results using 3D landmark analyses, and the evolutionary trajectories in early mysticetes will be extrapolated by including toothless species