Maxillary molars cusp morphology of South African australopithecines

The South African Plio-Pleistocene sites where large numbers of fossil hominid specimens have been discovered in the last 20 years are Sterkfontein, Swartkrans and, most recently, Drimolen. Hominid specimens recovered from these sites have usually been attributed to A. africanus (from Sterkfontein), A. robustus (Swartkrans, Drimolen and Sterkfontein) and South African early Homo (Swartkrans, Drimolen and Sterkfontein). We recently started a research project aimed at characterizing cheek teeth cusp morphology of South African Australopithecinae employing digital photographs of their occlusal surfaces. In this paper an analysis of the basic metrical features of maxillary molar cusp areas and proportions of A. africanus and A. robustus is presented. We analyzed 92 permanent maxillary molar teeth of South African Australopithecinae. The main results suggest that: a) crown base areas of the three molars are broadly similar in A. africanus and A. robustus; b) significant differences between the two species in relative cusp areas are evident for the protocone of M1 (with A. africanus larger than A. robustus), the paracone of M1, and the protocone of M2 and M3 (with A. robustus larger than A. africanus); c) in the total crown area A. robustus shows the sequence M1<M2<M3 as previously described; d) in A. africanus the sequence observed is M1<M2>M3, as in living apes. This different sequence between A. africanus and A. robustus appears to be related mostly to differences in mesial cusp size, which in A. robustus shows a marked relative expansion from M1 to M3. Also, the variability in absolute cusp areas of the A. africanus sample seems to be related to the presence of specimens with notably large teeth.