Elliptical Fourier analysis of symphyseal shape in great ape mandibles

The midsagittal profile of the mandibular symphysis has served as both a ta xonomic marker and a phylogenetically salient character in debates over hom inoid evolution. Nevertheless, the utility of symphyseal shape as an inform ative attribute for paleobiological reconstructions is suspect. Quantificat ion of shape variation has proven to be particularly problematic; it has lo ng been recognized that conventional linear measurements (and the indices d erived from them), while replicable, summarize aspects of shape very poorly because of the vast amount of contour information that is lost in the proc ess. In this study, a type of Fourier analysis is applied to cross-sectional con tours of ape mandibles in order to provide a mathematical accounting of sha pe variation in a "global" sense; that is, by applying the "totality" of co ntour information in a comparative analysis. Shape variation in the mandibu lar symphysis is explored through the decomposition of coordinate data into elliptical Fourier coefficients. These coefficients are used to compute av erage taxonomic distances (ATD) among individuals of chimpanzees, gorillas, and orang-utans. The resulting shape-based distances are summarized via cl ustering (UPGMA) and ordination (principal coordinates analysis-PCO). Princ ipal coordinate scores are subjected to analysis of variance in univariate and multivariate designs; these data are also applied to discriminant funct ion analyses. Species and sex effects on morphology are statistically significant; howeve r, no significant interaction of these factors is indicated. This would see m to imply that patterns of sexual dimorphism are not distinct among great apes; to the contrary, within-species sex comparisons reveal that significa nt size and shape dimorphism is present only in Gorilla. Despite significan t size dimorphism in Pan and Pongo, significant shape differences between m ales and females are not apparent in these taxa. These results suggest that it is theoretically possible to sort taxa by a s ymphyseal shape criterion, but the discriminant function results suggest th at there still exists a large potential for error in assigning particular s hapes to a given species or sex. Thus, despite real shape differences among these species, the use of symphyseal shape as a character in species ident ification or in systematic arguments remains limited and problematic. (C) 2 000 Academic Press.