Australopithecine Anterior Pillars: Reassessment of the Functional Morphology and Phylogenetic Relevance

The australopithecine anterior pillars defined by Rak (The Australopithecine Face, New York: Academic Press, 1983) were re-examined in the fossil hominids of southern Africa. The structure and extent of this buttressing pillar was found to be variable among Australopithecus africanus and A. robustus specimens. A reduced anterior pillar was observed in Homo habilis, and a morphological equivalent can be discerned in modern specimens of H. sapiens. The anterior pillars and associated features can be viewed as a response to the occlusal forces of the entire anterolateral dentition, with a special affinity to the canine but limited functional relationship to the “molarized premolars. Furthermore, a functional assessment of the hominid masticatory biomechanics implies that the adaptations ofA. africanus are well within our expectations of a viable ancestor to the genus Homo and are not irrevocably derived toward a “robust” type of adaptation. Morphological features of the craniofacial systems provide us with vital and distinctive information on which to base hypotheses concerning human evolution. In a thorough description of australopithecine facial topography, Rak (1983) was able to identify some key biomechanical features of the australopithecine masticatory morphology. These features have been used in interpretations of masticatory function and further applied to phylogenetic systematics (Rak, 1983, 1985a-c; White et al., 1981). Among the most important structures of the australopithecine gnathic apparatus is the anterior pillar (Fig. 1). This is a column of bone extending up from the anterolateral dentition past the piriform aperture, giving the lateral nasal margin a rounded contour. Although there are certainly other relevant features, many of these, such as the “maxillary furrow’’ (Rak, 1983), are consequences or functional correlates of the well-developed anterior pillar. The anterior pillar complex will thus be the focus of this analysis, as in Rak (1985b). Key points from the Rak model of australopithecine facial morphology can be summarized as follows. The anterior pillars and their associated complex are seen as structural buttresses for the occlusal forces applied to the anterior dentition. Because these pillars are found among Australopithecus africanus and A. robustus, but presumably not in other hominid taxa, a conclusion may be drawn that they represent a derived specialization tending away from the lineage that led to the genus Homo. This specialization can be associated further with “molarization” of the premolars, possibly indicating a response to the increased load associated with greater premolar occlusal surface area (Rak, 1983,1985a-c). Analyses of the relative occlusal forces and tooth size gradients have direct bearing on interpretations of the facial buttressing systems. This has prompted a careful reassessment of the fossil evidence from southern Africa, with a special focus on the anterior pillar buttressing complex. The purpose of the research presented here is to test the hypothesis that the anterior pillar ofA. africanus is a morphological response to the stress created by an increased premolar size. Received February 4,1988; accepted October 19,1988. @ 1989 ALAN R. LISS, INC