Late Pliocene Semnopithecus fossils from central Myanmar: rethinking of the evolutionary history of cercopithecid monkeys in Southeast Asia

We here describe a new fossil species of Asian colobine monkey, Semnopithecus gwebinensissp. nov. from the Late Pliocene Irrawaddy sediments of the Gwebin area in central Myanmar. Extant Semnopithecus (Hanuman langur) is a relatively large, terrestrial colobine monkey known as one of the most adaptable non-human primates. It is widely distributed, mainly in the Indian subcontinent, from Pakistan in the west to Bangladesh in the east. However, in Myanmar Semnopithecus is not distributed but Trachypihtecus is, which is the closest relative to Semnopithecus. It is presumed that extant Trachypithecus pileatus, which is considered to be a hybrid of Semnopithecus and Trachypithecus from molecular biological studies, appeared in the Early Pleistocene as the result of hybridisation between the two genera. On the other hand, no fossil specimens of other cercopithecid monkeys, such as Macaca, Trachypithecus or Rhinopithecus, all of which are commonly discovered from the Pleistocene cave sediments of South China, have been found from the Pliocene sediments in central Myanmar to date. The dissimilarity in the primate fauna between central Myanmar and South China suggests little faunal interchange between the two regions probably because of geographical barriers such as large rivers and high mountain ranges in the area.