Herbivore paleodiet and paleoenvironmental changes in Chad during the Pliocene using stable isotope ratios of tooth enamel carbonate

Chad is a key region for understanding early hominid geographic expansion i n relation to late Miocene and Pliocene environmental changes, owing to its location 2500 km west from the Rift Valley and to the occurrence of sites ranging in age from about 6 to 3 Ma, some of which yield fossil hominids. T o reconstruct changes in herbivore paleodiet and therefore changes in the p aleoenvironment, we measured the carbon and oxygen isotope composition of 8 0 tooth-enamel samples from three time horizons for nine families of Periss odactyla, Proboscidea, and Artiodactyla. The absence of significant alterat ion of in vivo isotopic signatures can be determined for carbon, thus allow ing paleodietary and paleoenvironmental interpretations to be made. While the results generally confirm previous dietary hypotheses, mostly bas ed on relative crown height, there are some notable surprises. The main dis crepancies are found among low-crowned proboscideans (e.g., Anancus) and hi gh-crowned rhinocerotids (Ceratotherium). Both species were more opportunis tic feeders than it is usually believed. This result confirms that ancient feeding ecology cannot always be inferred from dental morphology or extant relatives. There is an increase in the average carbon isotope composition of tooth ena mel from the oldest unit to the youngest, suggesting that the environment b ecame richer in C-4 plants with time. In turn, more C-4 plants indicate an opening of the plant cover during this period. This increase in carbon isot ope composition is also recorded within genera such as Nyanzachoerus, Cerat otherium, and Hex-aprotodon, indicating a change from a C-3-dominated to a C-4-dominated diet over time. It appears that, unlike other middle Pliocene hominid sites in eastern and southern Africa, this part of Chad was charac terized by very open conditions and that savanna-like grasslands were alrea dy dominant when hominids were present in the area.