Abstract
Molariform teeth have fascinated zoologists for as long as the field of evolutionary biology has existed, but few mammalian groups show as much morphological variation as hoofed mammals. Ungulate premolars and molars function together as the post-canine unit in grinding mastication. The degree of similarity of the premolars to the molars in crown complexity varies wildly across dietary ecologies and similar morphologies are refered to as molarized. However, the vast majority of dental complexity evolution research over the past 30 years has focused on molar crown morphogenesis evolution rather than interregional dental phenomena such as molarization. Dental crown complexity in vertebrates is controlled by signalling centers known as enamel knots in all regions of the jaw. In this study we tested whether applying current knowledge of enamel knot driven crown morphogenesis to shape covariation across the premolar molar boundary would inform potential mechanisms of molarization in hoofed mammals. We used 2D geometric morphometrics to study enamel-knot driven covariation at the lower premolar molar boundaries of 16 artiodactyl and 18 perissodactyls species. Phylogenetically informed modularity analyses were used to test several a-priori morphogenetic hypotheses describing different developmental interactions between the premolars and molars. Our results showed artiodactyls and perissodactyls significantly differ in their premolar molar boundary covariation caused by heterochronic shifts between premolar and molar development. To our knowledge, our study is the first to contribute a comprehensive yet accesible 2D morphometric method to produce heuristic results for further investigating the evolution of molarized premolars.