Accessory cusp expression in hominoid mandibular molars

Hominoid mandibular molars frequently display accessory cusps, particularly on the distal margin of the tooth (distal accessory cusp/C6) or the lingual margin of the tooth (lingual accessory cusp/C7). These presence, location and morphology of these cusps is utilised in studies of hominin systematics, where they are typically assessed at the enamel surface. However, studies of the enamel-dentine junction (EDJ) suggest that these traits may be more variable in development, morphology, and position than previously recognised. Here I score the expression of accessory cusps in extant apes (Pan, Gorilla; n = 106) and Plio-Pleistocene hominins (Paranthropus, Australopithecus, and Homo; n = 115) using a scoring procedure that considers the relationship between accessory cusps and the surrounding primary cusps. There are taxon-specific patterns in the EDJ expression of these traits; molars of Pan and Paranthropus typically have one or more distal accessory cusp but no lingual accessory cusps, while the opposite pattern is found in H. habilis M1s and M2s. Meanwhile Gorilla molars most often do not have accessory cusps. However, there are also a number of complicating factors. Some apparent accessory cusps at the enamel surface are represented at the EDJ only by ‘shouldering’ on the ridges associated with the main cusps, while other accessory cusps appear to have little or no EDJ expression at all. Shouldering features are highly variable in morphology, and in some cases are found closely associated with ‘true’ accessory cusps. The developmental basis of shouldering and enamel-only cusps is uncertain, and in particular, it is not clear whether these features are initiated by enamel knots. This means they may not be developmentally homologous to ‘true’ accessory cusps and would need to be considered separately when used for taxonomy or phylogeny. These results underline the importance of assessing the morphology of the enamel-dentine junction as well as the enamel surface wherever possible.