Taxonomic Implications of Molar Morphology Variability in Capuchins

Abstract

Tooth morphology has been widely used to infer taxonomic affinities. Both morphological and genetic analyses have revealed significant differences among capuchins, suggesting that two distinct monophyletic groups exist: the gracile capuchins (Cebus) and the robust capuchins (Sapajus). We developed a geometric morphometrics (GM) model to determine if the two groups also show distinct molar shapes and to explore the influence of size, phylogeny, and biogeography in shaping molar morphology. We characterized first and second molar crown shape variability in seven species representative of the two genera (Cebus albifrons, C. olivaceus, Sapajus apella, S. robustus, S. libidinosus, S. nigritus, and S. xanthosternos), using two-dimensional (2D) and three-dimensional (3D) GM. The results showed that 2D GM discriminated the two groups better than 3D GM, possibly because it accounted for cusp position and crown contour, but not cusp height, which may be a useful trait for inferring adaptive foraging ecology but presents a risk of homoplasy. In addition, the presence of a phylogenetic signal in the first molar shape (2D) is likely to reflect similarity to the ancestral condition and provides evidence of gradual evolution of molar robustness in the robust clade. We suggest that the shape of the first molar is informative about phylogenetic affinities, whereas the shape of the second molar is more informative about biogeographic variability. However, molar shape similarities may be affected by convergent evolution, as environmental factors in different biogeographical regions may have a significant effect on molar morphology, as seen in the closely related capuchins.