Analysis of the dental morphology of Plio-Pleistocene hominids. III. Mandibular premolar crowns.

Accurate measurements were made of the overall size of both the crown and its components of 91 mandibular premolar teeth of early hominids. The shape of the crown outline and the fissure pattern, and the expression of four morphological traits, were also recorded. Non-dental criteria were used to allocate the specimens into four major taxonomic categories (EAFROB, EAFHOM, SAFROB and SAFGRA), approximating to the hypodigms of, respectively, A. boisei, H. habilis and Homo sp., A. robustus and A. africanus. Those specimens that could not be so allocated were regarded as 'unknown'. Intertaxonomic overall size differences were established for both the P3 and P4, with the latter showing little overlap in crown size between the three taxonomic categories usually associated with East African sites (i.e. EAFROB, EAFHOM and SAFGRA). Crown shape is a better discriminator between taxonomic groups for P3 than for P4, with the P3s of EAFHOM showing less buccolingual expansion than the other taxonomic categories. Cusp number, the location of the lingual cusp and the expression of the median longitudinal fissure, show systematic variation between the main taxonomic categories, with the 'robust' taxa being distinguished by additional distal cusps, and a more deeply incised median longitudinal fissure, and EAFHOM being peculiar in having a distally situated lingual cusp. Marginal grooves show more overlap in their incidence and expression between taxonomic categories. Both the 'robust' australopithecine taxonomic categories have relatively large talonids, apparently at the expense of the size of the buccal cusp. The relative talonid enlargement was greater for P3 than for P4, a conclusion which is at variance with previous published assessments. Investigation of the allometric relationships between relative talonid size and overall crown size in the pooled 'non-robust' taxonomic categories did not suggest that talonid enlargement was a simple consequence of a larger-size crown. The results of multivariate analysis demonstrate that the absolute areas of the main cusps and the talonid provide marginally the more effective discrimination between the main taxonomic categories than do the relative areas of the cusp components. The removal of the simpler effects of overall size reduces the differences between taxa, but does not eliminate them. The data for the four taxonomic categories were used as a reference framework for the investigation of the affinities of those teeth in the unknown category for which detailed data were available.(ABSTRACT TRUNCATED AT 400 WORDS)