Mandibular shape disparity and convergence in ichthyosaurs and toothed cetaceans

Numerous tetrapod lineages have independently evolved to live in the water during the Mesozoic and Cenozoic, offering some of the best-known examples of convergent evolution. In particular, modern toothed cetaceans are often compared to ichthyosaurs, a diverse clade of extinct marine reptiles that also evolved a 'fish-shaped' body plan with tail-propelled locomotion. Both are predominantly raptorial marine tetrapods with long evolutionary histories and good fossil records, yet surprisingly the ecological convergences and the macroevolutionary pathways behind them are poorly understood. This project aims to investigate convergences of ichthyosaur and cetacean skulls on similar morphologies and ecological functions. Here we present results of a preliminary analysis focusing on mandible shape from a sample of archaeocete and odontocete cetaceans and parvipelvian ichthyosaurs. Landmarks and semi-landmarks were placed onto photographs of specimens or 3D models made with a handheld scanner. The resulting coordinates were subjected to a principal components analysis in R to show mandibular shape disparity, with preserved stomach contents and tooth shape data used to correlate how this morphological variation might relate to ecological function. Up-to-date phylogenies can be superimposed to show convergences and trajectories of evolutionary change in the two groups through time. Using these ordination techniques, an adaptive landscape can be created to show which areas of the morphospace ('peaks') are colonised more frequently. These results will form part of the first detailed quantitative analysis of ecomorphological convergence between ichthyosaurs and cetaceans.