Petrous Orientation

A more coronal orientation of the petrous usually implies that the cranial base is relatively broad and short, as opposed to a longer and narrower base, which is emphasized by more sagittal orientation of the petrous. Numerous processes may be involved in the overall orientation of the petrous and the shape of the cranial base, including head-balancing requirements and differential patterns of encephalization and brain morphology. Functional requirements associated with the digastric muscle (digastric fossa) may also affect the overall orientation of the petrous, though this is likely to be of secondary importance.

Three character states are recognized: the primitive condition with a sagittally oriented petrous as observed in Pongo, and Gorilla, which is oriented to almost 90° (= 0); an intermediate condition, as observed in specimens of Pan, Sahelanthropus (Brunet et al., 2002), Praeanthropus, and Australopithecus (= 1) (M.C. Dean & Wood, 1982; Strait et al., 1997); coronally oriented petrous as observed in all other hominins preserving this feature (= 2) (M.C. Dean & Wood, 1981, 1982; M.C. Dean, 1986; Walker et al., 1986; B. Brown et al., 1993; Strait et al., 1997; Keyser, 2000). Brunet et al. (2002) describe petrous orientation in Sahelanthropus as being within the Pan, Australopithecus (Praeanthropus), and Ardipithecus range. Strait and Grine, however, suggest that Gorilla and Pan share a more sagittal orientation as compared to Praeanthropus. On examining material of Pan and Gorilla, it is suggested here that the condition in Pan is intermediate between the conditions observed in Pongo and Gorilla at one extreme and the later hominins at the other. Thus, Gorilla and Pongo alone are considered to have a sagittal orientation of the petrous bone, while Pan shares a similar intermediate condition to that observed in the early hominins, including Praeanthropus and Australopithecus.

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