Longitudinal study of cranial base shape changes in Macaca nemestrina.

The primate cranial base (CB) represents a complex irregular structure even when limited to two dimensions, making it difficult to describe quantitatively using the conventional metrical approach composed of angles and distances. Consequently, a curve-fitting procedure, elliptical Fourier functions (EFF), was utilized, which accurately defines the form of complex two-dimensional morphologies. A longitudinal Macaca nemestrina sample consisting of 14 females and 15 males was utilized. Lateral headfilms, 1 to 8 years, were available (275 headfilms). Because the x-rays were not always taken at precise one-year intervals, a number of headfilms were excluded, which reduced the sample to 181. The CB boundary outline was carefully traced onto acetate sheets and 54 points located. The observed points were digitized and used to compute size-standardized EFF's with 20 harmonics. From each EFF, a set of 150 expected distances to the CB boundary was generated using the centroid as an origin. Superimposition of the CB's on the centroid also provided a detailed picture of the relative shape changes with respect to that center. Statistically significant shape differences were found as a function of age and sexual dimorphism. These age changes consisted of a gradual anteroposterior lengthening with a concomitant narrowing in the superoinferior direction. Specifically, a lengthening of the dorsal clivus and an anterior migration of the hypophyseal fossa could also be discerned. Thus, Fourier descriptors provide a particularly powerful method for documenting, both visually and numerically, the shape changes of complex two dimensional morphologies.