Lithostratigraphic context of Oligocene mammalian faunas from Ulantatal, Nei Mongol, China

The Ulantatal area is well-known for its extraordinarily rich Oligocene mammalian fossils. The Ulantatal fauna was originally considered to be the representative fauna for the Chinese Land mammalian Age, Ulantatalian. However, the abundant fossils collected in the 1980s lack coordinates and/or detailed stratigraphic levels, and have been lumped together as either single, coeval fauna or grouped into three units. This lack of stratigraphic information hampers more precise biostratigraphic division and correlation of the faunas. Here we present a complete lithostratigraphic profile of the Ulantatal Formation with new fossil localities calibrated into the profile. Lithologically, the sequence shows a rather uniform pattern characterized by interbedded reddish to yellowish brown claystones and siltstones, with minor fine-grained sandstones. Preliminary biostratigraphic analysis shows that the Ulantatal Formation covers most of the Oligocene, and offers a long sequence and successive fossil records for understanding the evolution of mammal faunas after the critical Eocene/Oligocene transition.