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 in 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. To reconstruct changes in herbivore paleodiet and therefore changes in the paleoenvironment, we measured the carbon and oxygen isotope composition of 80 tooth-enamel samples from three time horizons for nine families of Perissodactyla, Proboscidea, and Artiodactyla. The absence of significant alteration of in vivo isotopic signatures can be determined for carbon, thus allowing paleodietary and paleoenvironmental interpretations to be made.While the results generally confirm previous dietary hypotheses, mostly based on relative crown height, there are some notable surprises. The main discrepancies are found among low-crowned proboscideans (e.g., Anancus) and high-crowned rhinocerotids (Ceratotherium). Both species were more opportunistic 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 enamel from the oldest unit to the youngest, suggesting that the environment became richer in C4 plants with time. In turn, more C4 plants indicate an opening of the plant cover during this period. This increase in carbon isotope composition is also recorded within genera such as Nyanzachoerus, Ceratotherium, and Hexaprotodon, indicating a change from a C3-dominated to a C4-dominated diet over time. It appears that, unlike other middle Pliocene hominid sites in eastern and southern Africa, this part of Chad was characterized by very open conditions and that savanna-like grasslands were already dominant when hominids were present in the area.