The 84th Annual Meeting of the American Association of Physical Anthropologists (2015)

A reassessment of the taxonomic validity of the Australopithecus sediba mandibles


1Department of Anthropology, University of Arkansas, 2Department of Archaeology, University of Cape Town, 3Department of Anthropology, Max Planck Institute for Evolutionary Anthropology, 4Department of Biological Sciences, Bronx Community College, City University of New York

March 26, 2015 3:15, Grand Ballroom C Add to calendar

The mandibular remains of Australopithecus sediba consist of two relatively complete specimens—MH1 (subadult) and MH2 (adult). The shape difference in their rami is marked, which the original describers link to ontogenetic and/or intraspecific variation. However, others argue that this difference exceeds the variation expected in a single species, placing MH1 in Australopithecus and MH2 in Homo. To address this debate, this study evaluates variation in A. sediba in the context of extant great ape and human ontogenetic variation, with the working null hypothesis that the amount and pattern of shape variation represented by MH1 and MH2 are consistent with intraspecific variation in extant species.

To test this hypothesis, two-dimensional sliding semilandmarks were digitized on mandibular rami of Gorilla, Pan, Pongo, Homo sapiens, MH1, and MH2. The missing MH2 coronoid was reconstructed using thin plate spline and regression techniques. Shape differences (measured using Procrustes distances) were calculated between pairs of extant individuals of similar ages to MH1 and MH2, and compared to the distances between MH1 and multiple reconstructions of MH2. Although the distances between MH1 and MH2 are large, they only exceed the 95% confidence interval of a single species (Pongo). Thus, the hypothesis that MH1 and MH2 belong to a single (albeit highly variable) species cannot be refuted. Notably, we do not observe significant ontogenetic shape differences between the age groups represented by MH1 and MH2 in any extant species. These results challenge previous interpretations that the difference between MH1 and MH2 is due to ontogenetic shape variation.