Does Developmental Explain Molar Complexity in Haplorrhine Primates?

Abstract Body : Taxa consuming higher proportions of structural carbohydrates in their diets require additional tools in the form of sharp cusps and crests to reduce these food items. Most research documenting these dietary adaptations to mechanically challenging foods focus on differences in occlusal topography in one tooth, typically the first (M1) or second (M2) molar. Previously developed models of tooth development, suggest that these changes in occlusal morphology occur in predictable ways based on the balance of activators and inhibitors during dental development. Recent work on Euarchontans (primates, colugos, and treeshrews) suggests that a developmental model of dental development, the inhibitory cascade (IC) model, explains not only the morphogenetic gradient in size along the molar row, but also changes in occlusal complexity. Support for the IC model comes from the linear change in molar complexity along the tooth row, which they note is disassociated from changes in relative molar size. The project presented here builds upon this work to assess conformation of molar complexity to the IC model in a sample of 21 haplorhine primates. Relative molar complexity follows expectations of the IC model, increasing linearly across this sample with taxa consuming more structural carbohydrates having relatively more complex third molars compared to taxa with softer diets. In contrast to previous work, both dietary groups deviate from model expectations with slopes significantly greater than 1 (p< 0.001). These results suggest that while the IC model may regulate molar occlusal complexity and size along the row, there are additional factors at play.