This character is an index of bi-porion divided by orbital height (chords). The method used for defining the three character states used here is described earlier (see character 2). The overall breadth of the cranial base is significant in terms of head support and will have an impact on the orientation of numerous other basicranial morphologies (e.g., tympanic plate and the petrous bone). All data for the fossil hominins has been taken from B.A. Wood (1991); data for the extant hominids is unpublished and collected by Cameron. The index for P walkeri (KNM-WT 17000) was generated from a cast. The mean index for the extant hominids (n = 122) is 3.27, with one standard deviation = 0.27. Thus, a narrow cranial base is defined by an index of less than 3.00, intermediate is between 3.00 and 3.54, and a broad cranial base is defined by an index greater than 3.54.

Only Sahelanthropus, with an estimated index of 2.83, has a narrow cranial base (= 0) (data from Brunet et al., 2002). Pongo with a mean index of 3.05 (n = 22), Gorilla with a mean index of 3.30 (n = 35), Pan with a mean index of 3.33 (n = 65), Australopithecus with a mean index of 3.26 (n = 2), and H. habilis specimen KNM-ER 1813 with an index of 3.33 are all characterized by an intermediate breadth of the cranial base (= 1). Finally, a broad cranial base (= 2) is seen in P walkeri specimen KNM-WT 17000 with an estimated index of 3.72, P boisei with a mean index of 3.59 (n = 2), K. rudolfensis specimen KNM-ER 1470 with an index of 3.63, and H. sapiens with a mean index of 3.65 (n = 8).

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