Developmental basis of primate mandibular molar crown patterning: an endostructural perspective

As a growing number of studies have recently implicated important developmental models and mechanisms in the cusp patterning and overall crown morphology of certain mammal taxa, it was essential to assess the relevance of these processes to the primate dentition, and their potential implications to studies of primate crown morphology. In doing so, this also allowed for the assessment of current primate crown nomenclature schemes, which growing evidence has suggested may be critically flawed. This thesis focused on an examination of the enamel-dentine junction (EDJ) of lower molars in a taxonomically broad sample of primate taxa to address these two concerns. This work represents the first attempt to gain a broad perspective of crown patterning across all primates at the EDJ surface, and from this, present a more appropriate and unified assessment of cusp patterning and nomenclature that acknowledges the important developmental processes responsible for cusp expression.

The first chapter of the thesis reviewed the literature associated with studies of EDJ morphology, the recent advances in developmental biology relevant to the mammalian dentition, and the current state of the tooth crown nomenclature. Chapter 3 assessed the multiple phylogenetic and developmental components that appear to be responsible for crown patterning in mammals, and considered their application and consequence to the study of primate crown morphology. Important examples of previously unrecognized

aspects of growth are introduced here, and considered within the context of these developmental models. Chapter 4 employed geometric morphometrics to examine the covariation between accessory cusp presence and other aspects of molar crown shape in a population of macaque lower second molars, and demonstrates that while current development models used to interpret variation in cusp patterning are broadly appropriate in macaque molars, they do not explain all manifestations of accessory cusp expression. Chapter 5 focused on the first comprehensive analysis of variation in cusp patterning on mandibular molars within the major primate clades and from this assessed the applicability of the current nomenclature schemes to each clade. Results reveal numerous new patterns of lower molar accessory cusp expression in primates, and highlight the frequent discrepancies between the expected patterns of variation inferred from the current literature and the new patterns of expected variation discovered in this study. Chapter 6 provides a discussion of the broader results of this dissertation within the context of our current understanding of primate tooth crown development.