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.

Molar development in common chimpanzees (Pan troglodytes)

“…The number of Retzius lines and cuspal enamel thickness were both found to be variable among cusp and molar types, which led to variation in cusp-specific crown formation times. Patterns of cuspal enamel thickness are consistent with other studies that have suggested a trend between buccal and lingual analogues, which may be due to functional differences (e.g., Reid et al, 1998a;Spears and Macho, 1998;Schwartz, 2000;Kono, 2004;Smith et al, in press). …”

“…Smith (2004) examined the correlation between developmental variables in this sample, in part to determine if size differences among cusps or molars may explain some of the apparent variation. Bicervical diameter was used as a size scalar, as it has been demonstrated to have a positive relationship with tooth size (and by inference, body mass) (e.g., Martin, 1983;Shellis et al, 1998;Schwartz, 2000;Grine, 2002). Several variables showed positive allometry, including periodicity, number of Retzius lines, and crown formation time.…”

Variation in hominoid molar enamel thickness

“…However, Schwartz (2000a) suggested that the majority of published data on enamel thickness in modern humans are derived from European populations, which may not be representative of the species mean and may underestimate variation. A more recent investigation of enamel thickness in disparate modern human regional populations found values similar to those reported for European populations, with low levels of variation within and among populations (Grine, 2002(Grine, , 2005.…”

“…Another measure that has been employed as a scalar in enamel thickness studies, the bi-cervical diameter (measured between buccal and lingual cervices in cross-sections), may be better suited for standardizing size differences among taxa. Schwartz (2000a) and Grine (2002) recently employed bi-cervical diameter as a scalar for body size (although scalars are probably not necessary for intra-specific comparisons). While bi-cervical diameter has not been directly examined in individuals of known body size, Grine (2002) demonstrated a strong positive association between dentine area and bi-cervical diameter, and Shellis et al (1998) suggested that the former scales isometrically with body size among primates.…”

“…However, samples of each species were very small and the majority of molar type values were determined from single teeth. In particular, the issue of increasing enamel thickness from first to third molars within non-human primates remains unresolved (Schwartz, 2000a). If a pattern of increase can be shown, then values derived from different molars serve to increase the variance of combined tooth samples.…”