Variation in Mandibular Postcanine Dental Morphology and Hominin Species Representation in Member 4, Sterkfontein, South Africa

The hominin fossils from the Member 4 deposit at Sterkfontein, South Africa are most commonly attributed to Australopithecus africanus. However, a number of studies have suggested that they represent more than one species, although there is no consensus among those who recognize two (or more) taxa in this sample as to the allocation of individual specimens. We examine crown size and proportional cusp areas of the mandibular postcanine teeth, which constitute the bulk of the Sterkfontein Member 4 assemblage, to determine whether the degree of variation in it exceeds that of a single, highly dimorphic hominid species, Gorilla gorilla. The Lewontin CV ratio test, the Fligner-Killeen test and ordination of taxonomic distances via multidimensional scaling were employed to evaluate the degree of variation in the fossil and recent samples at each premolar and molar position. Sterkfontein (and combined Sterkfontein and Makapansgat) sample variation is significantly greater than that of the gorilla with regard to crown areas of the P₃ and P₄, relative P₃ metaconid size, and the MD diameter of the P₄. While the difference in P₃ metaconid size variability can be attributed to functional differences in this tooth between Gorilla and Australopithecus, the exaggerated premolar size variation at Sterkfontein defies such explanation. Those instances in which the Sterkfontein molar sample exhibits significantly greater variability are related to the expression of accessory cuspulids (C6 and/or C7) and the protostylid. Because these same features vary in incidence and expression in other fossil hominin species and among modern human populations, it would seem imprudent to attribute such variation to taxonomic heterogeneity in the Sterkfontein assemblage. Variation in Sterkfontein premolar crown size is intriguing and, perhaps in concert with the size variation that has been documented for Sterkfontein M²s, may hint at taxonomic heterogeneity. However, it may also be related to temporal heterogeneity if the Sterkfontein Member 4 deposit comprises a substantial time aggregate, as has been suggested. Indeed, time and heightened levels of sexual dimorphism have been cited in explanation for exaggerated variability in other fossil hominin assemblages that are interpreted as representing a single species. In this light, our results do not necessarily contradict the hypothesis that a single, polymorphic species, A. africanus, is represented in the Sterkfontein Member 4 deposit.