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The presence of multiple Australopithecus species at Sterkfontein Member 4, South Africa (2.07-2.61 Ma), is highly contentious, and quantitative assessments of craniodental and postcranial variability remain inconclusive. Using geometric morphometrics, we compared the sacrum of the small-bodied, presumed female subadult Australopithecus africanus skeleton Sts 14 to the large, alleged male adult StW 431 against a geographically diverse sample of modern humans, and two species of Pan, Gorilla, and Pongo. The probabilities of sampling morphologies as distinct as Sts 14 and StW 431 from a single species ranged from 1.3 to 2.5% for the human sample, and from 0.0 to 4.5% for the great apes, depending on the species and the analysis. Sexual dimorphism and developmental or geologic age could not adequately explain the differences between StW 431 and Sts 14, suggesting that they are unlikely to be conspecific. This supports earlier claims of taxonomic heterogeneity at Sterkfontein Member 4.

The discovery of the nearly complete Plio-Pleistocene skeleton StW 573 Australopithecus prometheus from Sterkfontein Member 2, South Africa, has intensified debates as to whether Sterkfontein Member 4 contains a hominin species other than Australopithecus africanus. For example, it has recently been suggested that the partial skeleton StW 431 should be removed from the A. africanus hypodigm and be placed into A. prometheus. Here we re-evaluate this latter proposition, using published information and new comparative data. Although both StW 573 and StW 431 are apparently comparable in their arboreal (i.e., climbing) and bipedal adaptations, they also show significant morphological differences. Surprisingly, StW 431 cannot be unequivocally aligned with either StW 573 or other hominins from Sterkfontein commonly attributed to A. africanus (nor with Paranthropus robustus and Australopithecus sediba). This finding, together with considerations about the recent dating of Plio-Pleistocene hominin-bearing sites in South Africa and palaeoecological/palaeoclimatic conditions, raises questions whether it is justified to subsume hominins from Taung, Makapansgat and Sterkfontein (and Gladysvale) within a single taxon. Given the wealth of fossil material and analytical techniques now available, we call for a re-evaluation of the taxonomy of South African Plio-Pleistocene hominins. Such an endeavour should however go beyond the current (narrow) focus on establishing an A. africanus-A. prometheus dichotomy.