Foramen Magnum Position

This character is defined by the position of basion, relative to the bi-tympanic line. This feature relates to cranial base length as well as the configuration of the tympanic. Following Strait et al. (1997), four character states are recognized. Pongo, Gorilla, and Pan define the primitive condition with basion positioned posterior to the bi-tympanic line (= 0). Ardipithecus, Praeanthropus, Australopithecus, P. walkeri, H. ergaster, and H. sapiens are all defined by "anterior migration" of basion to be aligned with the bi-tympanic (= 1) (T.D. White et al., 1995; Strait et al., 1997; Strait & Grine, 2001). Brunet et al. (2002) consider the position of the foramen magnum in Sahelanthropus to be similar to that observed in Ardipithecus, though they define the relative position from the bi-carotid chord. As such, it is allocated to the same condition as Ardipithecus. The next character state is defined by a variable state, as defined by H. habilis alone, because basion either is aligned with the bi-tympanic (KNM-ER 1813) or is placed anterior to it (= 2) (KNM-ER 1805). The last character state is defined by P. boisei and P. robustus, with basion placed anterior to the bi-tympanic (= 3).

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