Taxonomic implications of basicranial variation in Australopithecus africanus

Although it was discovered 85 years ago, Australopithecus africanus remains a source of contention among paleoanthropologists. Uncertainty about the fossils taxonomic unity has resulted in controversy about their place in hominin phylogeny. This work addresses their taxonomy through application of three-dimensional morphometrics followed by analysis of their patterns of variation in traditional morphological characters. This sequential approach lends more support to the conclusions than would either technique alone. The cranial base was selected as the focus of the analyses because it preserves well and is likely to capture taxonomically-important variation. This inference is supported by the finding herein that the cranial bases of Pan troglodytes and Pan paniscus are morphometrically distinct. Morphometrically, the sample of basicranial specimens typically assigned to A. africanus is shown here to be slightly more variable than is a broad pooled-species sample of Pan crania. The fossils\u27 pairwise Procrustes distances were compared to the distribution of similar distances within that Pan sample, and their percentile scores in its distribution were ordinated to reveal underlying patterns in their variation. This indirect approach to the comparisons permitted more fossils to be analyzed together than would otherwise have been possible. In these comparisons, several A. africanus specimens are shown to have distinct morphometric shapes in their basicrania. Of these, Sts 19, Sts 25, and Stw 580 also have distinct patterns of traditional morphological characteristics. The consistency with which these three specimens are distinguished indicates that they are likely to belong to a taxon other than A. africanus, but the fragmentary nature of Stw 580 renders this conclusion more tentative for that particular specimen. These findings do not support the hypothesis that the Sterkfontein and Makapansgat fossil assemblages represent different taxa, nor the hypothesis that the Sterkfontein sample contains approximately equal numbers of specimens from two distinct species.\u2