The curvature and components of the vault

The vault as a whole

The posterior part of the calvaria of Zinjanthropus is well filled and well rounded (pls. 9–12). From the external occipital protuberance, the posterior parieto-occipital plane rises steeply for a considerable distance before turning forwards over the summit of the vault. This striking feature, brought out in Fig. 1, was stressed by Leakey as the third of his twenty diagnostic criteria of Zinjanthropus (1959a, p. 492). However, Robinson (1960) claimed that this feature applied also to Paranthropus. In Fig. 1, Robinson's (1961) reconstruction of Paranthropus, based on the crushed specimen SK 48, has a remarkably similar parietooccipital contour to that of Zinjanthropus, whereas the earlier reconstruction of SK 48 by Broom and Robinson (1952, p. 11) had a very different parietooccipital contour from that of Zinjanthropus. As none of the specimens of Paranthropus is sufficiently undistorted to permit the contour in this region to be reconstructed accurately, it seems very likely that the Zinjanthropus-like contour in Robinson's later (1961) reconstruction has been influenced, at least subconsciously, by the intact parieto-occipital contour of Zinjanthropus. Robinson's (1960) claim that the steep parieto-occipital plane occurs as well in Paranthropus may therefore be discounted, at least until more intact cranial material is discovered. Australopithecus (Sts 5) has a more evenly-curved parieto-occipital surface.

Part of the full rounded contour of the vault is contributed by the high, steep, parietotemporal walls (pls. 9 and 11).