MORPHOLOGICAL VARIATION OF FOSSIL MERCENARIA SHELLS FROM WARM AND COLD CLIMATES OF THE PLIO-PLEISTOCENE ALONG THE US MID-ATLANTIC COASTAL PLAIN

The study of fossil bivalves provides valuable information on the biological consequences of ancient climatic and environmental changes in a context that can be useful for conserving modern bivalve populations in the face of climate change. The hard clam, Mercenaria, is a commonly used archive in paleoclimate and archaeological studies along the US Mid Atlantic Coastal Plain. Moreover, it is an economically valuable resource in the U.S. fishing industry. Therefore, understanding the impact of climate change on its shell growth is critical. Here, we investigate whether shell shape differs in fossil Mercenaria shells from the mid Piacenzian Warm Interval (MPWI) and subsequent early Pleistocene cooling collected from deposits in the US Mid-Atlantic Coastal Plan. Our hypothesis is that there is a greater morphological variation between shells from different climate states than populations within the same climate state. We employ traditional morphometrics to measure lengths, angles, and ratios of physical characters (n = 9) in 72 right-valved Mercenaria shells from North Carolina and Virginia. North Carolina localities include Robeson Farm from the Duplin Formation (n=30) of MPWI age and James Register Quarry from the Waccamaw Formation (n=15) of early Pleistocene age. Virginia localities include Fleet’s Mill and Lieutenant’s Run from the Yorktown Formation (n=13) of MPWI age and Edenhouse Landing, Gomez Pit, and Mt. Gould from the Chowan River Formation (n=14) of early Pleistocene age. Results based on ANOVA and Kruskal-Wallis scores show significant differences at 95% confidence among 4 of the characters: height, pallial sinus length, pallial sinus angle, and commissure thickness. Future work will investigate biological and environmental influences to explain these differences.