CHANGES IN SIZE AND MORPHOLOGY IN VENERICARDIA (BIVALVIA: CARDITIDAE) THROUGHOUT THE EARLY CENOZOIC OF THE U.S. COASTAL PLAIN

With global climate shifting at a rapid and unpredictable pace, researchers need additional tools to assess the effects these changes will have on modern organisms. The fossil record can be used to document how taxa change in response to a variety of climatic events, over a range of time scales. The bivalve genus Venericardia, which is abundant and well-preserved throughout the U.S. Coastal Plain during the early Cenozoic, is particularly useful for such a study. The goal of this study is to quantify trends in size, morphology, and morphological diversity of venericards from the early Cenozoic and to determine whether a relationship exists between these variables and global and regional temperature changes.

To quantify changes in body size and morphology, landmark data were compiled from 875 venericard specimens representing approximately 50 species. Data from Eocene venericards were compiled from previous studies; this study focused specifically on Paleocene, Oligocene, and Miocene species. Specimens obtained from museum collections were identified to the species level and photographed, and the resulting photos were used to collect landmark coordinates. Landmarks consist of 14 different homologous and pseudo-homologous points from both interior and cross-sectional views of right valves. Body size trends over time were quantified by calculating centroid size for each specimen and average centroid size for each species. To quantify shape changes over time, the landmark coordinates were transformed into Procrustes coordinates to remove any variation due to rotation or size. Principal components analysis was then used to create a morphospace, which facilitated comparison of taxonomic, temporal, and geographic differences in shell shape. To quantify morphological diversity over time, variance and mean pair-wise dissimilarity metrics were used. Trends in these variables were compared to global and regional temperature reconstructions. Preliminary results indicate a decrease in body size across the Paleogene, and a potential increase in globosity over the temperature decrease at the Eocene-Oligocene boundary.