Nuchal Plane Orientation

This feature will obviously be affected by the development of the nuchal muscles, including the rectus capitis posterior (major and minor) and the obliquus capitis superior. These muscles are involved in head rotation and elevation. As in the inflation of the mastoid, this character may also be influenced by differential patterns of locomotion (see earlier). Of secondary importance in the development of this feature will be the degree/pattern of cranial base angulation. Following Kimbel et al. (1984) and Strait et al.

(1997), this character is defined by the angle between inion-opisthion chord length and the Frankfurt horizon. All character states have been taken from Strait et al. (1997) and Strait and Grine (2001).

Three states are recognized: steeply inclined, angle >60° (= 0) as observed in Pongo, Gorilla, and Pan; intermediately inclined, angle between 60° and 45° (= 1), as observed in Praeanthropus; and weakly inclined, angle <45° (= 2), as observed in Australopithecus, all species of Paranthropus, and Homo as well as K. rudolfensis. Unfortunately, post-depositional distortion of Kenyanthropus specimen KNM-WT 400000 does not enable this character to be defined for this genus. While no data were directly available for Sahelanthropus, Brunet et al. (2002) state that the nuchal plane is near horizontal; thus, it is coded as the same condition observed in most hominins.

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