Variation in early hominin temporal bone morphology and its implications for species diversity

Temporal bone morphology features prominently in discussions of fossil hominin taxonomy and phylogeny. However, the complex morphology has led to different ways of interpreting features and, as a result, different conclusions regarding systematics. Here we use temporal bone anatomy and geometric morphometric techniques to ask how much disparity exists among early hominin temporal bones and whether levels of intra- and interspecific variation among fossil hominins are consistent with those among modern apes and humans. Using 3D ectocranial landmarks, Euclidean distances based on Procrustes coordinates were determined for all pairwise comparisons among 15 fossil specimens representing Australopithecus species and early Homo. These were then compared to distributions of intraspecific pairwise comparisons for five great ape and human samples, and interspecific comparisons for ten species pairs. Overlap between intra- and interspecific differences is pronounced in the extant as well as the fossil sample. On the whole, differences between fossil specimens matched the central tendency of differences between extant ape species or genera. In some cases the differences among fossils can also be found within highly variable extant species, even when the fossils are commonly recognised as different species. We conclude that the overall level of disparity among hominin specimens supports arguments that the clade is relatively speciose, but the pronounced overlap between intra- and interspecific variation suggests that classifications based largely on morphometrics would be unreliable.