Most fossil hominin species are sampled with spatial, temporal or anatomical biases that can hinder assessments of their paleodiversity, and may not yield genuine evolutionary signals. We use new fossils from the Kromdraai (Unit P) and Drimolen sites (South Africa) to provide insights into the paleodiversity of the Lower Pleistocene robust australopith, Paranthropus robustus. Our focus is the morphology of the temporal bone and the relationships between size and shape (allometry) of the semi-circular canals (SCC), an aspect that has not yet been investigated among southern African australopiths. We find significant size and shape SCC differences between P. robustus from Kromdraai, Drimolen and Swartkrans. This site-related variation is consistent with other differences observed on the temporal bone. P. robustus from Kromdraai Unit P is distinctive because of its smaller temporal bone and SCC, and its proportionally less developed posterior SCC, independently of age and sex. We emphasize the importance of allometry to interpret paleodiversity in P. robustus as either the consequence of differences in body size, or as yet unknown factors. Some features of the inner ear of P. robustus represent directional selection soon after its origin, whereas the size and shape variations described here may result from evolutionary changes.

Most fossil hominin species are sampled with spatial, temporal or anatomical biases that render difficult the assessments of their paleodiversity, and may not yield genuine evolutionary signals. We use new fossils from the Kromdraai (Unit P) and Drimolen sites to provide insights into the paleodiversity of Paranthropus robustus and to assess the possible occurrence of “robust” features in Australopithecus africanus. Our focus is first, the shape allometry of the semi-circular canals (SCC), an aspect that has not yet been investigated among southern African australopiths, and second, the morphology of the temporal bone. We find significant differences in P. robustus SCC shape between the sites of Kromdraai, Drimolen and Swartkrans, with some allometric trends. This site-related variation is consistent with other differences in the morphology and overall size of the temporal bone. P. robustus from Kromdraai Unit P is distinctive because of its smaller adult temporal bone and its smaller SCC size combined with its proportionally less developed posterior SCC, independently of age and sex. This SCC pattern is interpreted as either the consequence of allometric scaling of body size, or as a yet unknown ecological/functional diversity in P. robustus. Combined with the occurrence of plesiomorphic features of its temporal bone, the P. robustus palaeodiversity may result from either an adaptive radiation subsequent to the origin of this species, or phyletic evolution.