A geometric morphometric assessment of hominoid crania: conservative African apes and their liberal implications

This study examined the cranial affinities of all extant hominoids using 3D geometric morphometric analysis. A least squares Procrustean superimposition was used to eliminate differences due to location, orientation, and size. Because of a persistent correlation between centroid size and shape variation, an allometric size adjustment was also applied to these data. Phenetic affinities were then examined through a battery of multivariate statistical analyses. Results of this study indicate a strong affinity between Hylobates and Gorilla; Pan is also similar to these genera, while Pongo and Homo are each very different. The autapomorphic morphologies of orangutan and modern human crania have been well established from previous studies. The similarity between Hylobates and Gorilla, however, has important implications for studies of hominoid morphology. First, these results suggest that African ape crania--and particularly those of Gorilla--retain an overall morphology that is conservative among hominoids. Secondly, this similarity suggests that character coding of cranial features may tend to overestimate the degree of polymorphism among extant apes. This study concludes that allometry may play a greater role in the morphogenesis of hominoid cranial variation than has been previously thought. While this problem likely has negligible impact on systematic studies of extant hominoids, it seriously affects our ability to place fossil taxa within a phylogenetic framework.