A new lower jaw of Siamopithecus eocaenus from the Late Eocene of Thailand

We describe here a nearly complete lower jaw of Siamopithecus eocaenus from the Late Eocene of Peninsular Thailand. It displays several new characters, such as the symphysis, the horizontal branch, part of the vertical branch and most of its anterior dentition. The symphysis is nearly vertical and bears most characteristics of that of derived anthropoids but is unfused. Incisors were not preserved. A large canine, P/2 alveolus and P/3-M/3 were preserved. P/2 is rooted and reduced. The morphology of P/3 and P/4 is characteristic of that of anthropoids, with an oblique position in the tooth row, a labially displaced anterior root of P/3 and P/4 and the presence of a lingually developed metaconid on P/4. The premolars display a primitive stage of anthropoid evolution. This discovery confirms the anthropoid status of Siamopithecus, its attribution to the family Amphipithecidae and demonstrates the importance of Asia in the evolution of anthropoid primates.