CT-based study of internal structure of the anterior pillar in extinct hominins and its implications for the phylogeny of robust Australopithecus.

The phylogeny of the early African hominins has long been confounded by contrasting interpretations of midfacial structure. In particular, the anterior pillar, an externally prominent bony column running vertically alongside the nasal aperture, has been identified as a homology of South African species Australopithecus africanus and Australopithecus robustus. If the anterior pillar is a true synapomorphy of these two species, the evidence for a southern African clade of Australopithecus would be strengthened, and support would be given to the phylogenetic hypothesis of an independent origin for eastern and southern African "robust" australopith clades. Analyses of CT data, however, show that the internal structure of the circumnasal region is strikingly different in the two South African australopith species. In A. africanus the anterior pillar is a hollow column of cortical bone, whereas in A. robustus it is a column of dense trabecular bone. Although Australopithecus boisei usually lacks an external pillar, it has internal morphology identical to that seen in A. robustus. This result supports the monophyly of the "robust" australopiths and suggests that the external similarities seen in the South African species are the result of parallel evolution.