Mandibular postcanine dentition from the Shungura Formation, Ethiopia: Crown morphology, taxonomic allocations, and Plio-Pleistocene hominid evolution

Abstract

Over 200 hominid specimens were recovered by the International Omo Expedition of 1967-1976. Despite the fragmentary nature of this primarily dental collection, these hominid remains represent a major body of evidence about hominid evolution in eastern Africa during the 2-3 myr time period. Our analysis of the Omo dental collection is based on a large comparative sample of 375 quantifiable mandibular postcanine teeth of A. afarensis, A. africanus, A. aethiopicus, A. boisei, A. robustus, and early Homo. A total of 48 isolated mandibular premolars and molars of the Omo collection spanning the 2-3 myr time period is sufficiently preserved to allow reliable serial allocations and intertaxon comparisons and is the object of study in this paper. We present taxonomic identifications of these teeth and seven other mandibular specimens preserving 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 material to ensure accuracy of categorical evaluations. First, a combined metric and morphological evaluation was conducted to allocate each Omo tooth to either robust or nonrobust categories. Further taxonomic affinities were then examined. Our results indicate that nonrobust and robust lineages cooccur by circa 2.7 myr. We consider the Shungura robust specimens from Members C through F to represent A. aethiopicus. A significant phenetic transformation occurs at circa 2.3 myr, with the mosaic emergence of the derived A. boisei morphology across Member G times. Characterization of the East African nonrobust lineage is more difficult because of the comparatively subtle morphological differences seen among the dentitions of A. afarensis, A. africanus, and early Homo. The earlier Members B and C nonrobust specimens are difficult to evaluate 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. Homo sp. indet. At present, there is no indication of multiple species in the Omo nonrobust sample at any time horizon. The 2-2.4 myr Omo nonrobust specimens exhibit some similarities to the stated Homo "rudolfensis" condition in size and morphology and are likely to represent the ancestral condition of the genus Homo. The bearing of these results on interpretations of early hominid evolution and diversification is considered.

Geometric morphometric analysis of the crown morphology of the lower first premolar of hominins, with special attention to Pleistocene Homo

“…The H habiIis s. 1. group included the African Pliocene Table 1 List 0 f the specimens included in this analysis Taxonomical assignments of the isolated specimens from Zhoukoudian, Sangiran, and Shungura Formation fo llow Wei denreich (1937), Grine and Franzen (1994), and Suwa et al (1996), respectively. These assignments, based mainly upon the mor phology of the teeth (and also upon their geographical and chronological context), can be supported by our results.…”

“…"�,, 1t1,, of the archaic groups. included in this and not ences among African spl2ClmE�ns small sizes of these groups in our have been defined as derived with respect to hominin (e.g., Wood and 1987;Suwa, 1988;Suwa et aL, 1996; and Wood, and this our own results, .. II-I , ... ,..,,· . .... .…”

“…Recently, many dental studies based on image analysis of the occlusal morphology of fossil hominins (e.g., Bailey, 2004;Bailey and Lynch, 2005;Martinan-Torres et al, 2006;Gamez-Robles et al, 2007;Moggi-Cecchi and Boccone, 2007), non human extant primates (e.g., Bailey et al, 2004;Pilbrow, 2006;Hlusko et al, 2007), and recent modern human populations (e.g., Harris and Dinh, 2006;Perez et al, 2006;Bernal, 2007) have been published. Classical morphometric methods applied to image analyses (measurement of diameters and cusp areas) have demonstrated that African robust and non-robust groups differed in their P3 morphology (Wood and Uytterschaut,1987;Suwa et al, 1996). We have adopted geometric morphometric methods based on Procrustes superimposition techniques (Rohlf and Slice, 1990;Bookstein, 1991) to examine the morphological affinities among the hominin species included in this study.…”

Hominin lower second premolar morphology: evolutionary inferences through geometric morphometric analysis

“…Selection appears to have favoured larger occlusal areas in Pliocene hominins (Townsend et al, 1990;Suwa et al, 1996;McCollum and Sharpe, 2001). Interestingly, the modifications in the internal occlusal arrangement through the Pleistocene are generally related to a decrease in the occlusal polygon area (Table 3).…”

“…Large-toothed genera such as Australopithecus and Paranthropus (Wood and Uytterschaut, 1987) have subrectangular or ovoidal P4s (Fig. 6) with a disto-lingual bulge and mesially displaced main cusps (Genet-Varcin, 1962;Wood and Uytterschaut, 1987;Townsend et al, 1990;Suwa et al, 1996). In contrast, modern humans have small P4s (Wood and Abbott, 1983;Bermúdez de Castro, 1993;Bermúdez de Castro and Nicolás, 1995) with more circular outlines and more centrically located main cusps (Genet-Varcin, 1962).…”