C4 EXPANSION IN INNER MONGOLIA DURING THE LATEST MIOCENE AND EARLY PLIOCENE

Expansion of C4 grasses in the latest Miocene and Pliocene has been widely documented around the world. This ecological change has profoundly affected the global terrestrial biosphere. However, the spatial and temporal variations in C4 expansion are still not well understood. Here we present initial results of carbon isotope analyses of fossil mammalian teeth from central Inner Mongolia. Our samples represent a diverse group of herbivorous mammals including deer, elephant, rhino, horse and giraffe, and range in age from latest Oligocene to the present time. The δ¹³C values of tooth enamel samples of Mid-Miocene age or older, with the exception of two 13 Ma rhino samples of Hispanotherium tungurense, were all less than –8.0‰ (PDB), indicating that there were no C4 grasses present in their diets and thus probably no C4 grasses in the ecosystems of the central Inner Mongolia prior to ~9 Ma. However, tooth enamel samples of younger ages have δ¹³C values ranging from –9‰ to –5‰, indicating that herbivores in the area had variable diets ranging from pure C3 to mixed C3-C4 vegetation during the time interval of ~7 to 4 Ma. The presence of C4 grasses in herbivores' diets (up to ~50% C4) suggests that C4 grasses were a significant component of the local ecosystems in the latest Miocene and early Pliocene. Today, C3 grasses dominate grasslands in Inner Mongolia. The retreat of C4 grasses from this area may be driven by regional climate change associated with tectonic changes in central Asia as well as global climate change.