CONNECTIONS BETWEEN PALEOGENE CLIMATE CHANGE AND EVOLUTION: VENERICARD BIVALVES FROM THE U.S. GULF COASTAL PLAIN

Although recent projections make it possible to model the short-term effects of global climate change, the fossil record provides the only documentation of long-term biotic response. The Paleogene of the U.S. Gulf Coastal Plain provides an ideal system for exploring possible links between climate change and molluscan evolution. This study focuses on the effects of a series of warming and cooling events, including the Paleocene-Eocene Thermal Maximum, the early Eocene Climatic Optimum, and the Eocene-Oligocene transition, on evolutionary change in venericard bivalves. These bivalves are abundant and well-preserved throughout the Gulf Coastal Plain and have been used to provide regional reconstructions of mean annual temperature and seasonality based on microsampling of shell bands.

To explore ecological and evolutionary changes in venericard bivalves across these intervals, we quantified venericard diversity and body size. We compiled first and last appearance data from the primary literature and field collections for 80 species and applied boundary-crosser metrics to quantify taxonomic diversity. Landmark data for over 900 specimens, representing 26 species, from museum and field collections, were used to calculate centroid size and to track shifts in body size through time. Global patterns of temperature change were then used to assess potential correlations among climate, diversity, and body size. A phylogenetic framework was reconstructed for 20 venericard and 5 outgroup species based on 70 morphological characters, and used to determine to what extent body size patterns were controlled by phylogenetic relatedness. Data on sampling, preservation, and environment were also compiled and assessed to determine their potential influence on temporal patterns. Preliminary results indicate that venericard body size decreased significantly throughout the Paleogene, tracking global temperature change. In contrast, venericard diversity peaked in the middle Eocene, decreasing significantly across the E/O extinction.