The oldest anthropoid primates in SE Asia: Evidence from LA-ICP-MS U-Pb zircon age in the Late Middle Eocene Pondaung Formation, Myanmar

The Late Middle Eocene Pondaung Formation, central Myanmar hosts the richest deposit of terrestrial mammals in SE Asia. The Pondaung Formation contains anthropoid primates, such as Eosimiidae, Amphipithecidae and the new Afrotarsiidae, plus adapiform primates and is a critical locality in discussions on anthropoid origins and biogeography. The sands of the Pondaung Formation were derived from the erosional unroofing of a dissected andesitic volcanic arc and deposited on the forested floodplains of a large tropical river. Previously, the age of the Pondaung Formation was estimated to be Middle to Late Eocene based on stratigraphic evidence, Late Middle Eocene (Bartonian) based on comparisons with mammals from North America and Europe, 37.2 ± 1.3 Ma and 38.8 ± 1.4 Ma based on fission track dating and 37.4-37.0 Ma based on questionable magnetostratigraphic correlations. Here, we report a new LA-ICP-MS, U-Pb age for zircons from a tuffaceous bed in the Pondaung Formation of 40.31 ± 0.65 Ma and 40.22 ± 0.86 Ma which is slightly older than the debatable magnetostratigraphic ages of 37-36 Ma and 38-39 Ma for the anthropoids from Egypt and Libya. Pending the acquisition of similarly reliable radiometric dates from all the North African and Asian sites, this new date provides support for an Asian origin for the anthropoids. Our new dates are close to the molecular clock date for the origin of the anthropoid primates and may provide a reliable calibration point for the molecular phylogenetic method.