Population systematics of chimpanzees using molar morphometrics

When dental morphological variation within extant species is used as a guideline to partition variation within fossil samples into species, the underlying assumption is that fossil species are equivalent to extant species. This is the case despite the fact that dental morphology, which is commonly used to differentiate fossil species, is rarely used to differentiate extant species. Aspects of external morphology, ecology, behavior, breeding patterns, and molecular structure that are used to delineate living species are generally not available for fossils. In this paper, the utility of dental evidence for sorting fossil samples into species is evaluated by testing whether molar occlusal morphology is capable of sorting populations of Pan into the species and subspecies already well-established by nondental evidence. The dentitions of 341 chimpanzee individuals, sampled from regions throughout equatorial Africa, were sorted into 16 populations using rivers to demarcate the boundaries between populations. Digital-imaging software was used to measure 15 traits on the occlusal surface of each upper molar and 19 on each lower molar. After applying size adjustments, size-transformed and untransformed variables were subjected to discriminant analysis, with separate analyses carried out for each molar type. Results indicate that populations of Pan troglodytes and Pan paniscus are well differentiated at all molar positions. Populations of P. t. verus are distinct from other populations of P. troglodytes. Populations of P. t. troglodytes and P. t. schweinfurthii show close dental similarity. A distinct population is recognized at the Nigeria-Cameroon border, indicating the presence of P. t. vellerosus. The concordance between the patterns of diversity recognized by this study and other molecular and nonmolecular studies indicates that paleontological species that are similar to species of Pan in terms of size and patterns of diversification may be differentiated using molar morphology.