Taxonomic and functional aspects of the patterning of enamel thickness distribution in extant large-bodied hominoids

Abstract

One of the few uncontested viewpoints in studies of enamel thickness is that the molars of the African apes, Pan and Gorilla, possess "thin" enamel, while Pongo and modern humans possess varying degrees of "thick" enamel, even when interspecific differences in overall body or tooth size are taken into account. Such studies focus primarily on estimates of the total volume of enamel relative to tooth size (i.e., "relative" enamel thickness), as this is thought to bear directly on questions concerning dietary proclivities and phylogenetic relationships. Only recently have studies shifted focus to examining differences in the distribution of enamel across the tooth crown, i.e., the patterning of enamel thickness, as this may contribute to more refined models of tooth function and dietary adaptations in extant hominoids. Additionally, this feature has been suggested to be a reliable indicator of taxonomic affinity in early hominins, though no study has specifically addressed whether species-specific patterns exist among known phena. The aims of this paper were to test more explicitly whether enamel thickness patterning provides valuable taxonomic, functional, and/or phylogenetic information for maxillary molars of large-bodied extant hominoids. A series of seven linear enamel thickness measurements was recorded in the plane of the mesial cusps in cross sections of a total of 62 maxillary molars of P. troglodytes, G. gorilla, P. pygmaeus, and H. sapiens to estimate the patterning of enamel thickness distribution. Results from a discriminant function analysis reveal that, overall, this trait reclassifies extant hominoid maxillary molars with 90% accuracy: 100% of extant Homo, 75. 0% of Pongo, 83.3% of Pan, and 66.7% of Gorilla are reclassified correctly, indicating that this feature possesses a strong taxonomic signal. Furthermore, differences in the structure of the enamel cap are evident among hominoids: modern humans differ from Pongo in possessing proportionally thicker enamel in areas of the crown associated with shearing activity; Pan molars are better designed than those of Gorilla for generating a greater component of crushing/grinding loads. Thus, African ape molars are structurally dissimilar, even though they are both considered to belong to a morphologically homogeneous "thin-enameled" group. Simple developmental mechanisms can be invoked to explain the sometimes subtle differences in the achievement of adult morphology. For instance, human and orangutan molar cusps possess a similar degree of enamel thickness, but the possibility exists that despite similarities in morphology, each species follows a different sequence of secretory activity of enamel to achieve the final, albeit similar, degree of enamel thickness. Such a finding would suggest that the shared possession of "thick" or "thin" enamel among species may be phylogenetically uninformative, as it would not represent a developmental synapomorphy.