Controversial evidence hints that complex life might have emerged hundreds of millions of years earlier than previously thought—and possibly more than once

Hoofed mammal premolars show a range of occlusal crown morphology from molariform to caniniform, and the position of taxa on this spectrum can be described as the relative molarization of the premolars. Molarized premolars function together with the molars in grinding mastication in which these unique premolars appear. The degree of molarization varies across dietary ecologies, which has led to cheek tooth morphology being designated as an important contributor to dietary predictions in extant and extinct taxa. Recent research into mammalian occlusal cheek tooth patterning have found independent patterning mechanisms of the premolars and molars. A research gap exists in understand how molarization of the premolars has occurred so frequently in hoofed mammals if these dental regions are independent in their patterning. In this study, we tested the application of the patterning cascade model to the lower premolar-molar boundary in hoofed mammals using a geometric morphometrics framework. We used 2D geometric morphometrics to study occlusal cuspid covariation at the lower p4-m1 boundaries of 16 artiodactyl and 18 perissodactyl species. Phylogenetically informed modularity analyses were used to test alternate a priori hypotheses originating from evolutionary, developmental, and functional considerations of cheek tooth morphogenesis. Our results showed artiodactyls and perissodactyls differ significantly in their p4-m1 boundary covariation patterns, which we hypothesize could be caused by heterochronic shifts between premolar and molar development. To our knowledge, our study is the first to contribute a comprehensive yet accessible 2D geometric morphometric method to further investigate the evolution of molarized premolars.

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