Background/Question/Methods  
Current global warming has been documented to alter species distributions, abundances, interactions, and the timing of seasonal activities. However, long-term effects of warming on mammals are less understood. The fossil record provides a long-term record from which the effects of past global warming can be assessed.  Previous work has suggested that mammalian responses to interglacial warming were generally minor. Here, we use stable carbon and oxygen isotopes preserved in fossil teeth to document the magnitude of mammalian dietary shifts and ancient floral change during geologically documented glacial and interglacial periods during the Pliocene (~1.9 million years ago) and Pleistocene (~1.3 million years ago) in Florida. We first compare carbon isotope values and evaluate the following hypotheses: 1) dietary niches, inferred from the mean and breadth of carbon isotope values, remained consistent with interglacial warming; and 2) the proportion of taxa consuming C₄ vegetation decline with increased pCO₂ levels.  Bulk oxygen isotope samples from all taxa sampled document changes in relative humidity between the glacial and interglacial sites, while carbon and oxygen isotope data from serially-sampled horse teeth clarify how relative seasonality has changed with warming.  All mammalian taxa present within the orders of Artiodactyla, Perissodactyla, and Proboscidea, representing eight families, were sampled for stable isotopes and compared both within and between fossil localities.  

Results/Conclusions
Our data demonstrate increased aridity, increased C₄ grass consumption, inter-faunal dietary partitioning, increased isotopic niche breadth of mixed feeders, niche partitioning of phylogenetically similar taxa, and decreased seasonality with warming. These data show that global warming resulted in dramatic vegetation and dietary changes even at lower latitudes (~28°N). Lower pCO₂ levels typical of glacial periods were also not sufficient alone to support abundant C₄ floral environments, with C₄ grasslands instead expanding during the interglacial period. Our results question the use of bioclimatic models that predict the long term decline and extinction of species in response to warming, based on the assumption that niches are conserved over time. These findings have immediate relevance to clarifying possible biotic responses to current global warming in modern ecosystems.