Patterns of molar variation in great apes and their implications for hominin taxonomy

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Abstract

In studying the nature of variation and determining the taxonomic composition of a hominin fossil assemblage the phylogenetically closest and thus the most relevant modern comparators are Homo and Pan and following these, Gorilla and Pongo. Except for Pan, however, modern hominids lack taxonomic diversity, since by most accounts each one is represented by a single living species. Pan is the sister taxon to modern humans and it is represented by two living species. As such the species of Pan have greater relevance for studying interspecific variation in fossil hominin taxonomy. Despite their relatively impoverished species representations Pan troglodytes, Gorilla gorilla and Pongo pygmaeus are, nevertheless, represented by subspecies. This makes them relevant for studying the nature of intraspecific variation, in particular for addressing the question of subspecies in hominin taxonomy. The aim of this study is to examine the degree and pattern of molar variation in species and subspecies of P. pygmaeus, G. gorilla, P. troglodytes and P. paniscus. I test the hypothesis that measurements taken on the occlusal surface of molars are capable of discriminating between species and subspecies in commingled samples of great apes. The results of this study are used to draw inferences about our ability to differentiate between species and subspecies of fossil hominins. The study samples include P. t. troglodytes (n = 152), P. t. verus (n = 64), P. t. schweinfurthii (n = 79), G. g. gorilla (n = 208), G. g. graueri (n = 61), G. g. beringei (n = 30), P. p. pygmaeus (n = 140), and P. p. abelii (n = 25) . Measurements taken from digital images were used to calculate squared Mahalanobis distances between subspecies pairs. Results indicate that molar metrics are successful in differentiating between the genera, species and subspecies of great apes. There was a hierarchical level of differentiation, with the greatest separation between genera, followed by that between species within the genus Pan and finally between subspecies within species. The patterns of molar differentiation showed excellent concordance with the patterns of molecular differentiation, which suggests that molar metrics have a reasonably strong phylogenetic signal. Pan troglodytes troglodytes and P. troglodytes schweinfurthii were separated by the least dental distance. P. troglodytes verus was separated by a greater distance from these two, but on the whole the distances among subspecies of P. troglodytes were less than among subspecies of G. gorilla and P. pygmaeus. The dental distance between G. g. gorilla and G. g. graueri was greater than that observed between P. troglodytes and P. paniscus. With size adjustment intergroup distances between gorilla subspecies were reduced, resulting in distances comparable to subspecies of P. troglodytes. A contrast between size-preserved and size-adjusted analyses reveal that size, sexual dimorphism and shape are significant factors in the patterning of molar variation in great apes. The results of this study have several implications for hominin taxonomy, including identifying subspecies among hominins. These implications are discussed.