NEOGENE ECOSYSTEMS AND MAMMAL DIETS IN EAST AFRICA

Biomarker records from deep sea cores document dramatic terrestrial ecosystem change swept across much of Africa during the late Neogene (~10 Ma), when the primary spread of C₄ grasslands began, culminating in the modern savanna ecosystems that characterize much of sub-Saharan Africa today. Tooth enamel records from terrestrial archives in eastern Africa show that many large herbivores began to shift their diets towards C₄ vegetation (e.g., tropical grasses) at 10 Ma, concurrent with the primary spread of grassy ecosystems. Recent isotopic and phytolith data now suggest that C₄ vegetation was present prior to 10 Ma and as early as ~21 Ma in eastern Africa, which has major implications for our understanding of climatic drivers of vegetation change and the evolution of mammals, including primates and early humans.

A relatively understudied dimension of these early Neogene ecosystems is the diet of large herbivores. If C₄ vegetation comprised a significant proportion of early Neogene biomass, it is reasonable to assume herbivores would eat it. Here, we evaluate the diets of early Neogene large herbivores through stable isotope analysis of ~650 fossil teeth from seven fossil sites in the Turkana Basin ranging in age from 29 to ~5 Ma. We also compiled ~850 previously published enamel isotope data from other sites in Africa spanning 37 to 5 Ma. For samples older than 10 Ma, 98% indicate C₃-dominated diets with the remaining 2% of samples indicating possible minor amounts (up to ~10%) of C₄ vegetation in herbivore diets. We find no evidence of C₄-dominated diets. Overall, C₄ vegetation was likely present, consistent with previous results, but our results indicate it was not a significant proportion of large herbivore diets and its ecological significance requires additional evaluation. Furthermore, our new enamel isotope dataset enables finer scale investigations of dietary niche-partitioning, body water variations, and hydroclimate within and between fossil sites.