Leaving the triangle: marine tetrapod tooth guilds using automated high-density 3d geometric morphometrics

Defining feeding guilds based on tooth morphology is an entire subfield in marine tetrapod science, which essentially started with a seminal paper by Judy Massare in 1987. However, these assessments have mostly been qualitative, subjectively placing teeth on a triangle using a series of criteria that draw both from tooth shape, rarely preserved gut content, and killing/hunting behaviour. Moreover, some of the data at the foundation of these guilds have proven to be debatable and there is an ever clearer need for a testable, quantitative framework to assess feeding guilds. We develop a novel protocol that incorporates the pseudo-landmarking technique into high-density geometric morphometrics procedures, sampling 3D surface models of tooth crowns automatically and densely (e.g. 2000 surface landmarks) after placing just 5 fixed landmarks on each tooth. This very dense sampling of shape permits full characterisation of crown shape, and we provide the first ever quantification of dental morphospace occupation among a sample of mosasaurs, ichthyosaurs, plesiosaurs, archaeocetes, and odontocetes with conical (i.e., non-multicuspid) teeth. A crushing-to-piercing transition is evident along the first axis of the PCA-based morphospace, while the presence and shape of carinae, as well as crown curvature, is captured by the second axis. This allows an efficient visualisation of tooth shape differences and definition of regions/guilds with just two axes that explain ca. 90% of the total variance. Because crown shape has a direct functional signal, our results provide important data to better understand how marine tetrapods evolved and functioned. Despite their gigantic size, the teeth of the largest marine macropredator ever, the physeteroid Livyatan melvillei, have crowns with an unremarkable shape, occupying a position close to the center of the morphospace. In fact, peculiar structures such as crown curvature or carinae are mostly recorded on medium-sized teeth, suggesting that a scaling factor is at play. Similarly, crown shapes that have rarely or never been evolved, either by certain groups or during certain time periods, also carries information. For example, the perfectly straight teeth of the Early Jurassic ichthyosaurian Temnodontosaurus platyodon with protruding flange-like carinae are unique and indicate neoichthyosaurians developed a unique pathway towards hypercarnivory, albeit only once. Further work will incorporate as many different taxa and tooth-shapes into the morphospace as possible to usher in a new, quantitative paradigm for understanding marine tetrapod feeding ecology.SEASCAP