Mastoid Process Inflation

The overall inflation of the mastoid process will be associated with the development of the sternocleidomastoideus muscle, which is responsible for bending the neck laterally, head rotation as well as flexing the neck, and drawing the head ventrally. As such, this feature is likely to be influenced to some degree by differing patterns of locomotion, for it relates directly to the positioning of the head in relation to the clavicle and sternum (points of muscle origin). Two patterns are identified here. The lateral surface of the mastoid is not inflated beyond the supramastoid crest viewed frontally (= 0); the lateral surface of the mastoid is inflated to be level with, or beyond, the supramastoid crest (= 1).

Almost all species preserving this morphology are defined by a reduction in this feature (see Olsen, 1981; B. Brown et al., 1993; Strait et al., 1997). While not totally preserved in Dryopithecus, specimen RUD 77 suggests that the mastoid was probably not strongly developed relative to the supraglenoid gutter (see Kordos & Begun, 1997). Only species within Paranthropus as well as H. habilis display lateral inflation of the mastoid, though this may also be the case for Sahelanthropus, which is described as being large and pneumatized (Brunet et al., 2002). While Strait and Grine (2001) recognize H. habilis as being variable in this feature (i.e., KNM-ER 1805 has lateral inflation, while the inflation observed in KNM-ER 1813 is aligned to the crest), we considered these character states to be similar and thus place them within state 1 (not recognizing a third "variable" character state).

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