Paleoenvironment of the late Miocene Shuitangba hominoids from Yunnan, Southwest China: Insights from stable isotopes

The Late Miocene witnessed significant changes in climate globally and was an important time in the evolution of hominoids. Here, we report the results of an isotopic study of Late Miocene mammalian teeth and freshwater shells from Shuitangba (Yunnan Province, China) – an important refugium for hominoids, and reconstruct paleoenvironmental conditions in the area. δ¹³C values of fossil enamel samples range from −15.5‰ to −1.0‰, with a mean of −11.3 ±2.3‰, and δ¹⁸O values vary from −14.2 to −4.6‰, averaging −9.7 ±2.2‰. Reconstructed diet-δ¹³C values for these fossil mammals indicate that although they fed mostly on C₃ plants, many of them consumed some C₄ vegetation. This suggests that the local ecosystem contained C₄ grasses but was dominated by C₃ plants. The intra-tooth δ¹⁸O variation of fossil herbivores is larger than what is found in modern herbivores. Reconstructed paleo-meteoric water δ¹⁸O_w values are on average lower than those inferred from modern samples and also lower than the average δ¹⁸O_w values of modern precipitation in the region. Similarly, δ¹⁸O values of fossil freshwater shells display a larger seasonal variation and are significantly lower than modern shells from Fuxian Lake in the same region. Thus, the δ¹⁸O data from both mammalian teeth and freshwater mollusk shells support a wetter climate, possibly with a stronger precipitation seasonality in the Late Miocene than today. Comparison of clumped isotope temperatures from fossil and modern shells suggests a mean annual temperature of ~15 to 16 °C in the Shuitangba area in the Late Miocene, which is ~3 to 4 °C higher than that of today. Taken together, our isotope results suggest that C₄ grasses existed in local ecosystems in the area, likely in patches of grasslands or wooded grasslands in a mostly forested environment in the Late Miocene when the local and regional climate was warmer and wetter than today. Comparison of the δ¹³C records from Yunnan and the Siwalik region suggests that C₄ biomass spread earlier in the Indian subcontinent on the southwest side of the Tibetan Plateau than in Yunnan on the southeast side of the Plateau. The transition from a relatively wet habitat to a more open and drier habitat is also more pronounced and begins earlier in the Siwalik region, relative to Yunnan. These regional differences in climate and ecosystem evolution may be linked to the unique growth history of the Himalayan-Tibetan Plateau.