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Abstract :
[en] 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.
