Three-dimensional geometric morphometric analysis of the intermediate cuneiform in anthropoids

Abstract

The primate foot exhibits substantial morphological variation associated with locomotor behaviour, substrate use, and phylogenetic history. Despite its central role in load transmission, stability, and mobility during locomotion, the intermediate cuneiform remains one of the least studied tarsal bones in comparative morphology. This study uses three-dimensional geometric morphometrics (3DGM) to investigate variation in intermediate cuneiform morphology among anthropoids and assess the extent to which shape variation reflects locomotor behaviour, functional adaptation, and/or phylogenetic relationships. The sample for this study consists of 403 intermediate cuneiforms across various anthropoid species and focuses specifically on the facets that articulate with the navicular and second metatarsal bones and their impact on intermediate cuneiform shape. Digital 3D models derived from previously acquired laser scans of primate intermediate cuneiforms were analyzed using 3DGM of landmark and semi-landmark data. Analyses were conducted using generalized Procrustes analysis (GPA), principal component analysis (PCA), and Procrustes analysis of variance (PERMANOVA). Across all analyses, the first few principal components captured a substantial proportion of total shape variation, with the first three components explaining over half of the total variance. The results demonstrated that intermediate cuneiform morphology largely reflects phylogeny rather than any simple relationship with locomotor behaviour alone. Closely related taxa generally clustered together, whereas more distantly related taxa with similar locomotor behaviours frequently occupied different regions of morphospace. Atelids and colobus monkeys, for instance, clustered at opposite ends of the morphospace, despite both being primarily arboreal quadrupeds. The Procrustes analysis of variance further supports the interpretation that variation in the intermediate cuneiform is structured by phylogeny as well as locomotor behaviour, as it indicated that taxonomy and locomotor behaviour both explain a substantial proportion of total shape variation among anthropoids. Atelids, in particular, have short and stout intermediate cuneiforms that are unlike any other anthropoid in this study. Gorillas (and colobus monkeys to a degree) also have distinctive intermediate cuneiforms that are tall and narrow in shape. In addition to strong separation from other taxa, gorillas exhibit unusually high levels of interspecific and intraspecific variation in intermediate cuneiform morphology, suggesting that this variation is likely caused by lineage-specific evolutionary history. Unexpectedly, modern humans and fossil hominins occupied a relatively central position within the morphospace rather than forming a highly distinct cluster and consistently overlapped or clustered near orangutans despite major differences in locomotor behaviour. Overall, humans and most other anthropoids follow a similar structural framework for intermediate cuneiform morphology, with only minor differences occurring in the shape of this bone between species. These results suggest that the intermediate cuneiform is morphologically constrained, with phylogenetic history acting as the primary driver of variation while locomotor behaviour contributes secondarily.