Morphological variation of the Australopithecus afarensis maxilla

Central to discussions about hominin diversity in the mid-Pliocene of eastern Africa is whether or not certain fossils should be attributed to Australopithecus afarensis, instead of representing separate species. Key to answering this question is a good understanding of the magnitude and pattern of intraspecific variation shown by A. afarensis. Given the importance of maxillary characteristics in species diagnoses of early hominins, we explored morphological variation among all nine sufficiently preserved A. afarensis maxillae from Hadar. After CT-based virtual reconstruction, these were analyzed in the comparative context of 448 extant hominine (modern human and African ape) maxillae, representing all currently recognized subspecies, large geographic areas, and both sexes. Maxillary morphology was captured by three-dimensional landmarks, and size and shape were examined using geometric morphometric methods. The main findings are that 1) A. afarensis has high degrees of size and shape variation compared with extant hominines, potentially linked with sexual dimorphism, 2) no allometry was found, despite the large size variation, 3) a temporal trend in maxillary size is suggested but not in shape, and 4) the inferred patterns of sexual dimorphism in form and shape are different from those observed in Homo sapiens, Pan, and Gorilla. These results provide a greater understanding of A. afarensis, enable quantitative comparisons with contemporary maxillae attributed to Kenyanthropus platyops, Australopithecus deyiremeda, and Australopithecus bahrelghazali, and can help evaluate variation in other Plio-Pleistocene hominins, such as those assigned to species of early Homo.