Paleoenvironments and paleoecologies of Cenozoic mammals from western China based on stable carbon and oxygen isotopes

Three main objectives in this study were: (1) to examine climate variability throughout the Late Cenozoic and test hypotheses regarding the development of C4 ecosystems and the dynamics of the Asian monsoons in NW China; (2) to reconstruct the diets, habitats, and paleoclimates of fossil rhinocerotoids from the Linxia Basin, Gansu, China; and (3) to examine paleodiets, paleoecologies, and paleoclimates of extinct taxa and test previous hypotheses regarding expansion of C4 grasses in SW China.;To examine climate variability in NW China throughout the Late Cenozoic and to test hypotheses regarding the development of C4 ecosystems and the dynamics of the Asian monsoons, the carbon and oxygen isotopic compositions of bulk tooth enamel samples from 158 fossil mammals from the Linxia Basin, ranging in age from 25 Ma to the present, were determined and serial carbon and oxygen isotopic analyses of 368 samples from 23 fossil and modern herbivore teeth were performed. The results indicated significant changes in the climates and diets of mammalian taxa from the Linxia basin, as well as in the seasonal patterns of diet and climate, over the last 25 million years. The bulk oxygen isotope data indicated an unstable climate in the Linxia Basin from 25 to 0.05 Ma and fluctuations in the oxygen isotope data throughout the entire sedimentary sequence were consistent with previous studies that indicated a general global warming trend from ∼26 Ma to 15 Ma and two major cooling phases during the Neogene. In addition, a positive delta18O shift in the data was similar in timing to a positive delta18O shift observed in fossils and paleosols from Pakistan and Nepal, suggesting a shift toward a drier and/or warmer climate on both the north and south sides of the Tibetan Plateau during the Late Miocene.;The diets, habitats, and paleoclimates of fossil rhinocerotoids from the Linxia Basin, Gansu, China, ranging in age from 25 to 2.5 Ma, were reconstructed based on bulk and serial carbon and oxygen isotope analyses of tooth enamel. Bulk isotope analyses of 47 rhino individuals representing 11 genera and serial analyses of 15 of those individuals were performed. In addition, the bulk C and O isotopic compositions of teeth from 5 9-Ma Chilotherium individuals from the nearby Tianshui Basin were determined. The results support many previous hypotheses inferred from taxonomy and cranial and limb morphology and offer new insight on the paleoecologies of some genera. The isotopic results support the following previous hypotheses: the Late Oligocene rhino Paraceratherium inhabited a forested environment, and the coexisting rhino Allacerops lived in a relatively open habitat and had a less specialized diet; the Early Miocene Hispanotherium grazed in open steppe territory, whereas the contemporaneous Alicornops had a more generalized diet in a forested environment; and the Late Miocene rhino Parelasmotherium grazed in an open steppe habitat. The data are inconsistent with previous inferences that the rhinos Acerorhinus and Dicerorhinus dwelled in forested environments. Instead, the results indicate that these two rhinos inhabited open steppe environments.;To examine paleodiets, paleoecologies, and paleoclimates of extinct taxa and to test previous hypotheses regarding a global expansion of C4 grasses, the carbon and oxygen isotopic compositions of 164 fossil herbivore teeth and 10 soil samples from six localities in Yunnan Province, China, ranging in age from ∼10 Ma to the present, were determined. The results reveal significant changes in the environments and diets of mammalian taxa from various regions of Yunnan Province over the last 10 million years. The carbon isotope results showed that the diets of mammals aged ∼2.5 to 1.75 Ma from Shangri La ranged from pure C3 to pure C4, while 1.7 Ma horses from Yuanmou had 0-50% C4 in their diets. The paleoenvironment of Zhaotong was more open and/or water-stressed than that of Kaiyuan at ∼4 Ma. Mammals living at both ∼8 and 7.5 Ma in the Lufeng region had very similar diets, habitats, and experienced similar climatic conditions. Increased C4 biomass after ∼2--3 Ma suggests a significant change in certain aspects of the 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 oxygen isotope compositions of ∼7.5 and 8 Ma mammals from Lufeng suggest very similar climatic conditions at both ages in that region. The oxygen isotope results show a positive shift after ∼8--8.5 Ma in the Yuanmou region, which is similar in timing to shifts observed in horses, rhinos, and deer from the Linxia Basin and in fossils and paleosols from Pakistan and Nepal, suggesting a shift toward a drier and/or warmer climate at the northeast, southeast, and southern borders of the Tibetan Plateau during the Late Miocene. (Abstract shortened by UMI.)...