Skull shape evolution in ichthyosaurs and cetaceans: detailed investigations into a classic example of convergence

Ever since they first made the transition to life on land around 350 million years ago, more than 30 lineages of tetrapods have reinvaded the water independently, filling important roles in aquatic ecosystems. The constraints that arose from living in water rather than air have forced the evolution of similar morphologies within these groups, making them some of the best-known examples of evolutionary convergence. In particular, modern day toothed cetaceans are often compared to the ichthyosaurs, a diverse clade of extinct marine reptiles which also evolved a ‘fish-shaped’ body plan with tail propelled locomotion. Both are groups of 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 and lack a thorough, quantitative framework. The goal of this project is to investigate convergences of ichthyosaur and cetacean skulls on similar morphologies and ecological functions.

We gathered a series of measurements and ratios that have direct ecological consequences (e.g. tooth and snout shape) from 2D and 3D preserved specimens. The initial dataset comprises ratios from 31 species of extinct and extant toothed cetaceans and 26 species of ichthyosaurs. These data were subjected to clustering and ordination techniques in R using a 50% completeness threshold. The results were corroborated with preserved stomach contents and modern ecological observations to reveal general convergence patterns within the two groups. These preliminary results will be investigated further in more detail by using them alongside analyses of 3D cranial and jaw landmarks. By combining these with the most up to date phylogenies for each group we can reveal parallel trajectories of convergence in marine tetrapods over time.