Abstract: Thousands of fossil teeth of Pleistocene orangutan were found in South China. In this study, we analyzed enamel thickness of 23 fossil teeth of Pleistocene orangutan from Guangxi by micro-CT scanning and three-dimensional reconstruction, and made some comparisons with other extant primates and modern humans, fossil apes and hominins, and discussed the issues of functional adaptability and taxonomy of enamel thickness of Guangxi fossil orangutan. The results are as followings. Enamel thickness of Guangxi fossil orangutan is not significantly related to the tooth size within the same tooth type, and there is no significant difference between maxillary and mandibular cheek teeth, and there is no significant difference with other fossil orangutans from different sites of Guangxi. Compared with other fossil apes, enamel thickness of Guangxi fossil orangutan is intermediate/thick. Both average enamel thickness (AET) and relative enamel thickness (RET) of Guangxi fossil orangutan are smaller than ones of Australopithecus and Paranthropus, and also smaller than Homo erectus, Neanderthal and early fossil Homo found in Africa and Europe. Compared with extant primates, enamel thickness of Guangxi fossil orangutan is thicker than most of monkeys and African apes obviously. Average enamel thickness of Guangxi fossil orangutan is a little thicker than extant orangutan but closer to modern human. Relative enamel thickness of Guangxi fossil orangutan is thinner than modern humans, and little difference with extant orangutan and both belong to intermediate/thick enamel. Concerning the taxonomy and phylogeny and diet adaptation of enamel thickness, we suggest that enamel thickness might be an intrinsic property that had evolved independently, and have a relationship with dental function. The intermediate/thick enamel thickness of Guangxi fossil orangutan maybe show that they had a relatively hard food adaptation.

Key words: Guangxi; Fossil orangutan; Enamel thickness; CT；Diet