Hominin diversity in the Middle Pliocene of eastern Africa: the maxilla of KNM-WT 40000

Abstract

The 3.5-Myr-old hominin cranium KNM-WT 40000 from Lomekwi, west of Lake Turkana, has been assigned to a new hominin genus and species, Kenyanthropus platyops, on the basis of a unique combination of derived facial and primitive neurocranial features. Central to the diagnosis of K. platyops is the morphology of the maxilla, characterized by a flat and relatively orthognathic subnasal region, anteriorly placed zygomatic processes and small molars. To study this morphology in more detail, we compare the maxillae of African Plio-Pleistocene hominin fossils and samples of modern humans, chimpanzees and gorillas, using conventional and geometric morphometric methods. Computed tomography scans and detailed preparation of the KNM-WT 40000 maxilla enable comprehensive assessment of post-mortem changes, so that landmark data characterizing the morphology can be corrected for distortion. Based on a substantially larger comparative sample than previously available, the results of statistical analyses show that KNM-WT 40000 is indeed significantly different from and falls outside the known range of variation of species of Australopithecus and Paranthropus, contemporary Australopithecus afarensis in particular. These results support the attribution of KNM-WT 40000 to a separate species and the notion that hominin taxonomic diversity in Africa extends back well into the Middle Pliocene.

Figure 1.

CT-based parallel-projected 3D reconstructions comparing the maxillae in lateral view of (a) A.L. 200-1 (reversed right side of cast, Australopithecus afarensis) and (b) KNM-WT 40000 (left side of original, Kenyanthropus platyops). The five landmarks are shown, together with the connecting wire frame used in figure 3 (see text for the abbreviations of the landmarks). The broken surface of the zygomatic process of KNM-WT 40000 facing laterally is not visualized to emphasize the outline compared with the equivalent morphology in A.L. 200-1. Scale bar, 10 mm.

Figure 2.

Distortion of KNM-WT 40000. (a) Anterior view, giving the midline (black line) to indicate the midface skewing, left nasospinale (ns), prosthion (pr) and the trajectory used to calculate the height expansion of the subnasal area (white line). (b) CT-based 3D reconstruction of the maxilla in superior view, showing the tooth roots inside the translucent bone. (c) A high-resolution sagittal CT image through the buccal roots of the left P³ to M² (orientation indicated by the black line in (b)). The thin black lines mark a longer crack through the premolar roots. (d) Lateral view of the left maxilla, showing the pattern of matrix-filled cracks highlighted by wetting with acetone. The five landmarks are shown as in figure 1. The trajectories along which crack widths were measured are given by lines with associated percentages of expansion (black line refers to the subnasal trajectory shown in (a)). (e) The right M² crown (M, mesial; B, buccal), with black lines marking the endpoint of cracks highlighted with acetone. The white lines indicate the match at the mesial end of the widest crack. The dark area on the mesiolingual corner is a strong shadow of the enamel more distally, rather than damage to the dentine. Scale bars, (a,b) 30 mm, (c,d) 10 mm and (e) 3 mm.

Figure 3.

Bivariate plots of PCs. (a) PC2 against PC1 and (b) PC5 against PC4 of the fossils samples. (c) PC2 against PC1 of the combined fossil and extant samples. KNM-WT 40000 (black dot) is corrected for distortion. The prefix KNM- of the Kenyan specimens is omitted, and an asterisk indicates subadults. Convex hulls are given for A. afarensis (solid line), A. africanus (dash-dot line) and P. robustus (dashed line), as well as in (c), the 95% confidence ellipses of these taxa and the extant species (solid line). The grey-shaded wire frames in (a,b) are defined in figure 1 and indicate the maxillary shapes represented at the extremes of the PC axes. See the main text for the percentage of variance represented by each PC.