3D Geometric Morphometric Analysis of Hylobatid Cranial Ontogeny: Implications for Interpreting the Evolutionary Hstory of Hominoid Cranial Growth

Research in hominoid cranial ontogeny has provided significant insight into the similarities and differences between apes and humans. Additionally, questions about within-species variability, allometry, and levels of sexual dimorphism in fossil hominoids are commonly addressed using the extant great apes as a comparative framework. However, this model is incomplete without the addition of the lesser apes, gibbons and siamangs. This analysis completes the hominoid record of cranial ontogeny with the addition of the Hylobatidae, and provides a full description of their cranial ontogeny and adult variation. Three-dimensional coordinates of 145 landmarks and 313 semilandmarks were measured on CT and surface scans from an ontogenetic sample of hominoid crania, comprising Pan, Gorilla, Pongo, Hylobates, and Symphalangus, with a cercopithecoid out-group, Colobus. After Procrustes superimposition, principal component analyses were computed in shape space. Regressions of shape coordinates on centroid size were used to assess within-group ontogenetic and static allometric trajectories. Results indicate that the shape changes during ontogeny in gibbons are similar to the shape changes previously reported for great apes. Genus-specific differences are already observable early in ontogeny, and the subsequent ontogenetic trajectories are almost parallel. Sexual dimorphism for both shape and size is found in Symphalangus adults, which was previously unobserved. Analysis of all taxa in the sample shows nearly parallel ontogenetic trajectories within the Hominoidea, which is consistent with previous studies. With the addition of hylobatids and Colobus, this analysis demonstrates that cranial ontogeny is highly conserved in the Catarrhini. Given the existence of this basic catarrhine growth trajectory, it should be possible in the future to predict fossil taxa morphologies at any stage of growth.Ph