Effects of the mid-Miocene Climatic Optimum on ecosystem structure and plant-animal interactions: A phytolith and stable isotope perspective

The mid-Miocene Climatic Optimum (MMCO, 17--14.75 Ma) is one of Earth's most recent, prolonged global warming events that is thought to have promoted ecological change across the globe. Although there is general agreement about the timing of broad-scale ecosystem changes associated with the MMCO, specific details of its influence on local paleo-communities remain obscure, as no studies have been able to characterize plant and animal responses in the context of long-term, local climate. However, new radiometric dates for the Railroad Canyon section (RCS; 22.9--15.2 Ma) of Idaho firmly establish this region as a critical field area for testing hypotheses about the biotic consequences of long-term global warming during the early--middle Miocene. The RCS is an exceptionally important sequence of rocks preserving a nearly-continuous record of animals and plants that inhabited the Northern Rocky Mountain, USA, during pre- and peak-MMCO times. Results from stable carbon and oxygen isotopes from fossilized mammalian-herbivore tooth enamel establish that local climate was consistently warm and dry through the early--middle Miocene, clearly decoupled from global climate change. Phytolith (plant silica) data indicate that the RCS was a mosaic of C3-grass dominated, open habitats and woodland patches during the early--middle Miocene, which pushes back the timing of when open-habitats spread in this region by approximately five million years. And lastly, stable carbon isotopes from tooth enamel indicate that horses and rhinos inhabiting the RCS consistently fed on C3 vegetation leading into the MMCO. Ultimately, these data have, for the first time, allowed us to develop a comprehensive picture of climate and ecosystem change in a single basin leading into the MMCO. These results highlight how crucial it is to study many different regions to gain a full understanding of the local impacts of climate change in deep time. Specifically, data from the RCS provide insight into the response of a high-elevation site located in the continental interior to global warming during the MMCO. Given that inland, high-elevation sites are generally under-represented in the fossil record, the RCS is an important field area that has the potential to provide a unique perspective on the climate, ecology, and evolution of ecosystems during the early--middle Miocene.