Bone by bone: phylogeny of the ape cranium

Abstract

There is perhaps no topic more richly investigated in physical anthropology than the phylogeny of our closest relatives: the hominoids. Studies have ranged from examining how qualitative and metric traits differ, variation in molecular structure and how cranial modules contribute to phylogenetic history. Despite this, there has been no single study comparing individual cranial bones and their effects on phylogeny. This study explores how allometric and non-allometric shape impact on phylogeny and whether phylogenetic methods and geometric morphometrics are compatible by comparing morphological phylogenies to the accepted molecular phylogeny. I conducted a suite of multivariate and geometric morphometric analyses to reconstruct the phylogenies of great apes (N=177) and small apes (N=57) from virtually generated three-dimensional cranial models. From these models, I analysed phylogenetic signal and the allometric and non-allometric shape of seven cranial bones. Results were surprising: almost all bones in the cranium provide strong phylogenetic signal support but this did not translate into the accepted phylogenies being reconstructed. The phylogenies express morphology dominated by robust superstructures with more gracile morphology forming a separate clade. This strongly suggests a dominance of morphologies dependent on robusticity. Researchers must be careful in interpreting fossil hominin taxonomy using geometric morphometrics for, as this study has shown, morphologies dependent on robusticity will cluster in phylogenetic trees but may not be congruent with the molecular phylogeny. Due to the nature of preservation, fossils will likely never have a complete molecular phylogeny and this study provides a warning for use of methods which are common-place now but carry unexpected results.