JAW CORRELATES OF DIET PROVIDE NOVEL INSIGHT ON THE ADAPTIVE RADIATION OF EARLY THERIAN MAMMALS

Therian mammals underwent an immense adaptive radiation and now comprise all but three genera of living mammals. However, there remains considerable debate over the timing and dynamics of the radiation, particularly as it relates to the Cretaceous-Paleogene (K-Pg) mass extinction event 66 million years ago (Ma). This is due in part to difficulties in examining patterns of ecological diversity in deep time, which requires ecological inferences from fossil morphologies. One source of paleoecological data is jaw morphology, which is expected to be associated with diet. Thus, to identify jaw correlates of diet that can be readily applied to fossil mammals, I first examine the association between jaw shap­e and diet in modern mammals. To this end, I apply phylogenetic comparative methods to linear jaw measurements and quantitative dietary data for a taxonomically diverse sample of mammals. Results indicate that the distance between the jaw joint and angular process (JAPr distance) is an especially powerful predictor of diet, increasing with greater herbivory and likely reflecting differences in jaw muscles sizes among dietary groups. Further, I compare the fit of evolutionary models to the JAPr distances and find strong evidence for the presence of unique selective regimes associated with herbivory and faunivory. Finally, I apply these findings to the fossil record by measuring the JAPr distance in therian jaws from the Cretaceous and Paleogene (145-23 Ma). Fossil patterns suggest that therians were small insectivores for much of the Cretaceous and experienced a rapid increase in dietary diversity 10-15 million years prior to the K-Pg boundary, coinciding with the ecological diversification of flowering plants. In addition, the diversification of herbivorous mammals accelerated after both the K-Pg mass extinction event and the Paleocene-Eocene Thermal Maximum (56 Ma), and these patterns may also be linked to evolutionary expansions in angiosperms (e.g., larger seeds and fruits) that provided novel ecological opportunities. Thus, this study demonstrates the strong correlation between jaw morphology and diet, and it offers new insight on early mammal evolution by suggesting a multi-step adaptive radiation.