QUANTIFYING CHANGES IN SHELL MORPHOLOGY OF CHESAPECTEN (BIVALVIA) ACROSS THE MID-PLIOCENE WARM PERIOD IN VIRGINIA

From approximately 3.3 to 3.0 Ma, mean sea surface temperatures rose to ~1.8°C to 3.6°C above pre-industrial temperatures in an event known as the Mid-Pliocene Warm Period (MPWP). These conditions provide a useful analog for climate conditions expected by the end of the 21st century. Studying fossils from this period can help us to better understand how biotas may respond to similar climate change in the next few decades. Chesapecten is a genus of benthic scallop that was abundant in U.S. Atlantic Coastal Plain sediments from the Early Miocene to the Early Pleistocene. This study aims to determine how shell size and morphology of the genus Chesapecten change across the intervals before, during, and after the MPWP, and to compare these changes to latitudinal differences in body size and morphology in modern-day scallops. Specimens were compiled from bulk samples archived at the Virginia Museum of Natural History (Martinsville, VA), as well as one field location. Museum specimens were measured from six localities (n = 18 specimens per locality), representing each of the four members of the Yorktown Formation in southeastern Virginia. An additional field sample was collected from the Rushmere Member of the Yorktown Formation at Carter’s Grove, Williamsburg, VA in September 2021. The largest shells were selected for measurement to avoid sampling juveniles whose morphology may differ due to allometric growth rather than climate change. Each shell was digitally photographed and measured for 35 morphological characters. A Principal Components Analysis was used to quantitatively assess changes in shell size and shape. The results of the study indicate that Chesapecten shell size decreased across the MPWP, then rebounded almost immediately after. This follows the same pattern as latitude-based size variation in modern Pectinidae. The total morphological variation within Chesapecten also decreased between the intervals before and after the MPWP.