Paleoecologies and paleoclimates of late cenozoic mammals from Southwest China: Evidence from stable carbon and oxygen isotopes

To examine paleodiets and habitats of extinct taxa and to understand long-term regional climate change, we determined the carbon and oxygen isotopic compositions of fossil herbivore teeth and soil samples from six localities in Yunnan Province, Southwest China, ranging in age from ∼10 Ma to the present. Although limited in spatial and temporal coverage, these initial results reveal significant changes in the environments and diets of mammalian taxa over the last 10 million years. Prior to 2–3 Ma, while most mammals examined had pure or nearly pure C3 diets, some individuals consumed a small amount of C4 grasses (up to 20% C4). Since then, C4 grasses became a significant dietary component for most herbivores as indicated by higher enamel-δ13C values in the Pleistocene Yuanmou Formation and at Shangri-La, most likely reflecting an increased C4 biomass in the region. The carbon isotope results show that the diets of mammals of ∼2.5–1.75 Ma from Shangri-La ranged from pure C3 to pure C4 while 1.7 Ma horses from Yuanmou had 0–70% C4 grasses in their diets. Mammals living at ∼8–7 Ma in the Yuanmou and Lufeng region had very similar diets and habitats, with similar climatic conditions. Increased C4 biomass after ∼3–4 Ma suggests a significant change in certain aspects of regional climate, such as increased seasonality of rainfall or an increase in seasonal drought and fires as these factors are important to modern grasslands. The data also show that unlike the Siwalik fauna in the Indian subcontinent, mammals in Yunnan on the southeast side of the Himalayan–Tibetan Plateau lived in an environment dominated by dense forests until ∼3–4 Ma. Nonetheless, both δ13C values of paleosol carbonates and fossil enamels indicate that C4 grasses were present in the Yuanmou region in the latest Miocene and Pliocene (∼8–3.5 Ma), likely in greatly dispersed, small patches of open habitats where the forest canopy was broken or on flood plains, and the C4 biomass increased significantly after ∼3.5 Ma. The oxygen isotope results from Yuanmou (Xiaohe Formation) show a positive shift after ∼8.5 Ma, which is similar in timing and magnitude to δ18O shifts observed in horses and rhinos from the Linxia Basin and in fossils and paleosols from Pakistan and Nepal, suggesting a shift toward a drier climate at the northeast, southeast, and southern borders of the Tibetan Plateau during the late Miocene. Taken together, the carbon and oxygen isotope data indicate a general drying of the local climate over time and a change from a largely dense-forest environment at ∼8 Ma to a more open environment with a mosaic of forests and grasslands after 3–4 Ma in the Yuanmou region. Intra-tooth δ13C and δ18O variations within individual fossil teeth from Yuanmou suggest a stronger seasonality of rainfall at ∼1.7 Ma than in the late Miocene. The spatial and temporal δ13C and δ18O variations observed in mammalian teeth from Yunnan likely reflect changes in regional climate and/or tectonics, but more data are needed to fully explore the significance of the regional patterns in the δ18O and δ13C data in relation to climate and tectonic evolution of the region.

► We examine diets and habitats of extinct taxa at six localities in SW China. ► Prior to ∼3 Ma, mammals fed predominantly on C3 plants in forested habitats. ► C4 grasses became a significant dietary component after 3.5 Ma. ► The increase in C4 biomass after ∼3.5 Ma was likely due to increased seasonal drought.