This is an index of basion-sphenobasion length divided by cranial base geometric mean (see Strait, 2001). This index gives a relative length of the basioccipital. This feature relates directly to facial prognathism relative to the cranial base. The authors did not calculate any of these indices, except for Pongo (n = 2) but did use the coding structure and taxa allocation of Strait (2001). The primitive condition is defined by a long basioccipital, as seen in Gorilla and Pan (= 0), an intermediate condition as calculated for Praeanthropus, Australopithecus, and P walkeri (= 1), and a short length for all other taxa preserving this feature (= 2) (Strait, 2001). Pongo, with an index of 0.20, was most similar to Gorilla with an index of 0.18, thus sharing the same condition as Gorilla and Pan.
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