ECOLOGICAL EFFECTS OF MIOCENE-PLIOCENE COOLING ON MOLLUSKS FROM THE EASTOVER AND YORKTOWN FORMATIONS OF SOUTHEASTERN VIRGINIA

Understanding faunal response to past climate change can help us to predict the impacts of current anthropogenically-driven warming. The fossil record provides an ideal opportunity to explore the effects of climate change on ecological communities. The goal of this research is to quantify changes in diversity and community structure, recorded in the molluscan assemblages of southeastern Virginia, that coincide with an interval of cooling across the Miocene-Pliocene boundary (approximately 5.3Ma). To explore the effects of this cooling, we compared community characteristics before and after the climate change. The Cobham Bay Member of the Eastover Formation, a silty sand, was deposited during the late Miocene warmer interval and records an abundant and diverse assemblage of subtropical mollusks. This contrasts with the more depauperate warm temperate fauna of the Sunken Meadow Member of the Yorktown Formation, which is early Pliocene in age. The two stratigraphic units are similar sedimentologically and paleoenvironmentally, allowing us to compare diversity while controlling for substrate.

We collected 16 bulk samples at two field localities, and supplemented this material with eight museum samples from four localities from the Virginia Museum of Natural History. At each outcrop, we described the sediment, measured stratigraphic sections, and collected latitude and longitude data using GPS. Field samples were standardized to 9kg, sieved with a 2mm mesh size and identified to species level. Abundance data were collected by counting hinges of bivalves and apices of gastropods. We quantified the following ecological variables: species composition, richness, evenness, abundance, and body size using shell length. Preliminary results suggest an increase in the origination or immigration of temperate species and an increase in the extinction or emigration of tropical species across the Miocene-Pliocene boundary. Rarefaction indicates that, even at sample sizes of 3000 individuals, species richness does not plateau with increased sampling for samples from either unit. Comparing sample size-standardized estimates of species richness across the boundary does demonstrate a decrease in diversity.