Tooth size and shape and their relevance to studies of hominid evolution.

Abstract

Teeth have the potential to provide evidence about both the patterns of diversity of fossil hominids and the functional adaptations of early hominid taxa. Comparative studies of dental function and the direct examination of wear patterns in fossil teeth are now providing data for testing hypotheses that major differences in dietary adaptations underlie lineage diversity in the early hominids. However, this review focuses on the contributions that dental evidence can make to hominid systematic studies. Attention is drawn to the value of tooth enamel as a morphological marker and the major contribution that teeth make to the hominid fossil sample. Systematic analysis of hominid remains must start with the identification of patterns of morphological variation. Only then can the taxonomic significance of the morphological differences be assessed and attempts made to link designated taxa in a phylogenetic scheme. The preliminary results of a detailed metrical survey of early hominid premolar and molar teeth are presented. As part of this study cusp areas of first mandibular molars were measured by planimetry. Analysis of these data, without any prior assumptions about taxonomic groups, has demonstrated that the major axis of variation separates the pooled sample into morphological subgroups. These methods provide a systematic and rigorous way of identifying patterns of tooth crown morphology and will allow a more objective assessment of the affinities of individual specimens. Fossil taxa are described in terms of both absolute and relative tooth size. If canine base area and molar crown area are considered there is considerable overlap between Australopithecus africanus and Australopithecus ( paranthropus) robustus whereas there is little or no overlap between the ranges of Australopithecus africanus and Australopithecus (Parnthopus) boisei . Differences in relative tooth size among fossil taxa are taken as an example of how to attack the problem of assessing the taxonomic significance of morphological differences. Analogues from modern primates are used to derive tooth-body size relations for three relative growth models. The results suggest that increases in body size are usually accompanied by a more rapid rate of increase in canine size than in molar size. This suggests that the relatively smaller canines of the ‘robust’ australopithecines are not the result of simple scaling, but represent the result of selection against an allometric trend. Preliminary results of a survey of the subocclusal morphology of fossil teeth are presented to indicate the potential of radiographic studies and to demonstrate that changes in root morphology can be correlated with crown shape and relative size.