MANDIBULAR POSTCANINE DENTITION FROM THE SHUNGURA FORMATION, ETHIOPIA- CROWN MORPHOLOGY, TAXONOMIC ALLOCATIONS, AND PLIOPLEISTOCENE HOMINID EVOLUTION

Over 200 hominid specimens were recovered by the International Omo Exp edition of 1967-1976. Despite the fragmentary nature of this primarily dental collection, these hominid remains represent a major body of ev idence about hominid evolution in eastern Africa during the 2-3 myr ti me period. Our analysis of the Omo dental collection is based on a lar ge comparative sample of 375 quantifiable mandibular postcanine teeth of A. afarensis, A. africanus, A. aethiopicus, A. boisei, A. robustus, and early Home. A total of 48 isolated mandibular premolars and molar s of the Omo collection spanning the 2-3 myr time period is sufficient ly preserved to allow reliable serial allocations and intertaxon compa risons and is the object of study in this paper. We present taxonomic identifications of these teeth and seven other mandibular specimens pr eserving tooth crowns. Metric analyses of this study include cusp area and crown shape variables taken on occlusal view diagrams. Nonmetric analyses were based on simultaneous observations of all relevant mater ial to ensure accuracy of categorical evaluations. First, a combined m etric and morphological evaluation was conducted to allocate each Omo tooth to either robust or nonrobust categories. Further taxonomic affi nities were then examined. Our results indicate that nonrobust and rob ust lineages cooccur by circa 2.7 myr. We consider the Shungura robust specimens from Members C through F to represent A. aethiopicus. A sig nificant phenetic transformation occurs at circa 2.3 myr, with the mos aic emergence of the derived A. boisei morphology across Member G time s. Characterization of the East African nonrobust lineage is more diff icult because of the comparatively subtle morphological differences se en among the dentitions of A. afarensis, A. africanus, and early Home. The earlier Members B and C nonrobust specimens are difficult to eval uate and are considered indeterminate to genus or species. Both molars and premolars from Members E through G exhibit phenetic similarities to the early Homo condition and are considered as aff. Home sp. indet. At present, there is no indication of multiple species in the Omo non robust sample at any time horizon. The 2-2.4 myr Omo nonrobust specime ns exhibit some similarities to the stated Home ''rudolfensis'' condit ion in size and morphology and are likely to represent the ancestral c ondition of the genus Homo. The bearing of these results on interpreta tions of early hominid evolution and diversification is considered. (C ) 1996 Wiley-Liss, Inc.