Analysis of the dental morphology of Plio-Pleistocene hominids. II. Mandibular molars--study of cusp areas, fissure pattern and cross sectional shape of the crown.

Accurate measurements of the absolute and relative size of individual cusps, the arrangement of the primary fissure system and the shape of coronal cross sections of the tooth crown have been used to investigate the pattern of variation in Plio-Pleistocene hominid mandibular molar teeth. Teeth were either grouped into one of six taxonomic categories or considered as individual cases. Univariate analysis of relative cusp areas shows that the two taxonomic categories of 'robust' hominids from East and Southern Africa have relatively small mesial cusps, but a relatively large entoconid and hypoconulid and Principal Component plots of the data show that the 'robust' categories can be distinguished on the basis of relative cusp size. Other evidence suggests that these differences are not likely to be the result of allometric phenomena. Fissure pattern was analysed using the X/Y coordinates of defined reference points. Patterns were compared by Procrustes analysis and the relationships between teeth contained in the resulting similarity matrix were portrayed using Principal Coordinates plots and a nearest neighbours table. The positions of the posterior fovea and the mesial longitudinal fissure were important for distinguishing taxonomic categories. The shape of the coronal profiles proved difficult to quantify, but there were consistent and distinct differences between the South African 'robust' sample and teeth included within the East African Homo category. When these results are combined with those of a previous study of overall crown size and the distribution of extra cusps, they allow the affinities of isolated teeth or contentious specimens to be assessed. For example, our results show that KNM-ER 1506 and 1802 are more similar to the East African Homo group than any other category, and they indicate that though SK 1587 and 1588 are small teeth, they nonetheless are closest to the South African 'robust' category in terms of relative cusp size, fissure pattern and crown profile shape. The closest affinities of the Taung First mandibular molars are also with the South African 'robust' sample.