A functional morphometric study of the posterior cranium

Neandertal crania are characterized by large prognathic faces, mastoid processes that appear to be small and non-projecting, and a long, low, broad and horizontally oriented nuchal plane. It has been argued that these features are functionally related; the large horizontally oriented nuchal plane enhances the ability of the nuchal musculature to balance and support the large and prognathic face, and the small appearance of the mastoids is engendered by the low position and horizontal orientation of the nuchal plane. The purpose of this thesis is to identify the functional and morphological implications of a large and horizontally oriented nuchal plane and to determine if the functional relationships that have been hypothesized to exist in Neandertals also exist in modern humans. The posterior cranial morphology of 114 male and 102 female crania representing three modern populations was examined. Seven morphological and functional complexes in the posterior cranium were identified by principal components analysis: occipital plane length; posterior vault size; nuchal plane orientation; nuchal plane length; mastoid size; occipital curvature; and posterior breadth. The nuchal plane can be regarded as one of the functional complexes of the posterior cranium, and both size and orientation are significant aspects of its morphology. The size and orientation of the nuchal plane appear to be related to the morphology of the mastoid region of the temporal and the size and prognathism of the face, but the pattern of relationships is only partially similar to the morphological pattern seen in Neandertals. The size of the nuchal plane, particularly the anterior breadth, appears to be functionally related to the size of the face and the lowness of the cranial base; as the face becomes larger and the cranial base inferiorly positioned the nuchal area becomes broader and the occipital more posteriorly projecting. There is no relationship between the orientation of the nuchal plane and the size or projection of the mastoid processes, and the the relationship between nuchal plane orientation and facial prognathism is quite different from the pattern seen in Neandertals. In modern humans a prognathic face appears to be associated with a more posteriorly, not inferiorly, oriented nuchal plane. In modern humans the size of the nuchal plane, particularly its breadth, appears to be functionally related to the size of the face and the lowness of the cranial base, but the orientation of the nuchal plane may be related more to the morphology of other areas of the posterior cranium than to the functional demands placed upon it by the nuchal musculature.