External Cranial Base Flexion

This feature is developmentally significant because it relates to a number of mosaic influences that will affect a number of morphological patterns, including facial orientation, neuro-orbital disjunction, postorbital constriction (and thus indirectly influencing the development of the tempo-ralis through the size of the temporal fossae), and development of the supraorbital torus, to mention just a few (see main text for further details). Given the usually poor preservation of the phenotypic landmarks in fossil specimens that are usually used to define cranial base flexure/extension, we can only partly define this feature by observing the degree of angula-tion of the external cranial base. The feature is defined here by the angle between the Frankfurt horizontal and the basion-hormion chord length; that is, the inclination of the basioccipital and the basisphenoid measured externally (see Strait et al., 1997; Strait, 2001). As in the measure of nuchal plane inclination, the character states and their allocation to taxa have been taken directly from Strait and Grine (2001).

Pongo, Gorilla, Pan, and P. walkeri are defined by the primitive condition of a flat external cranial base (= 0). Australopithecus is defined by a moderately flexed cranial base (= 1), while Paranthropus boisei, P. robus-tus, H. habilis, H. ergaster, and H. sapiens are defined by a strongly flexed anterior cranial base (= 2) (see Strait & Grine, 2001). For the same reason outlined by Strait et al. (1997), Praeanthropus was not scored because the specimen used by other researchers to define this species as displaying a weak inclination (e.g., Skelton & McHenry, 1992, and presumably D.E. Lieberman et al., 1996) is based on specimen AL 333-105, which is an infant.

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