1Institute of Human Origins, School of Human Evolution and Social Change, Arizona State University, 2Center for the Study of Human Origins, Department of Anthropology, New York University, 3New York Consortium in Evolutionary Primatology, NYCEP, 4Kunming Institute of Zoology, Chinese Academy of Sciences, 5The Kyoto University Museum, Kyoto University
April 22, 2017 , Acadia
Gibbons and siamangs (family Hylobatidae) are small apes inhabiting the rainforests of South and Southeast Asia. There are up to 18 extant species, apportioned among four genera (Hylobates, Hoolock, Nomascus and Symphalangus). Although hylobatids share a suite of dental and skeletal specializations that distinguish them from other apes, differences in hard tissue features within the Hylobatidae are extremely subtle, limiting our ability to identify fossil specimens at lower taxonomic levels. Here we quantified size and shape differences in the upper and lower molars of 279 extant hylobatids (913 molars), representing 13 species from all four genera. Cusp angles and areas were collected from digital photographs of the occlusal surface using image-processing software. Crown outlines were analyzed using geometric morphometrics methods and then submitted to discriminant analyses (DFAs) to determine differences between genera. These results were mapped onto phylogenetic trees to ascertain the role of phylogeny in hylobatid molar morphology. In the upper molars, Hylobates cusp angles appear to be highly derived. However, in the lower molars, it is Symphalangus that diverges from other hylobatids, with a relatively larger metaconid and hypoconid. The DFAs show significant differences in molar shape between hylobatid genera, although the phylogenetic component of this variation is not significant for any tooth. LM1 outline in particular provided a highly accurate classification (~93% accuracy). Considering that the hylobatid fossil record consists primarily of isolated teeth of unknown generic and specific affiliation, our results have implications for understanding hylobatid biogeography and evolutionary relationships.