Paleodietary change and its implications for aridity indices derived from δ18O of herbivore tooth enamel

A large body of research provides reconstructions of ancient water deficit-the difference between potential evapotranspiration and precipitation-using an aridity index derived from the δ18O values of tooth enamel from evaporation sensitive (ES) and evaporation insensitive (EI) mammalian herbivores. The former are typically ruminants that obtain most of their water from leafy dicots (the δ18O of their enamel reflects evaporation of leaf water), whereas the latter are non-ruminants dependent on surface water (the δ18O of their enamel reflects local meteoric water). Use of the index requires that fossil taxa are correctly classified as ES or EI. This study examines how paleodietary change can influence such classifications. In modern ruminant ungulates, dependence on drinking water is related to the proportion of grass versus dicots in the diet. This means that paleodietary change in ruminant ungulates should be associated with shifts in drinking behavior, altering enamel δ18O values and biasing the aridity index. Such bias is evident in estimates of water deficit derived from the enrichment of enamel δ18O in fossil Tragelaphini relative to Hippopotamidae from East African fossil sites spanning the last ~ 4.4 Myr. Elevated grass consumption (measured by enamel δ13C) in the former is associated with reduced enrichment of enamel δ18O relative to the latter, translating to lower estimates of water deficit. In conducting such analyses of fossil ungulates, the diet of those ungulates must be screened to ensure consistency with the assumptions of the method.