THE EFFECTS OF THE MID-PLIOCENE WARM PERIOD ON MOLLUSK DIVERSITY RECORDED IN THE YORKTOWN FORMATION (SOUTHEASTERN VIRGINIA)

The marine fossil record offers important insights into how biodiversity responds, both ecologically and evolutionarily, to specific environmental changes. The Mid-Pliocene Warm Period (MPWP) was an interval of marked climate change, with global temperatures warming by approximately 3 °C over the course of 300 ky. The purpose of this study is to explore how marine molluscan diversity may be impacted by sustained global warming and ocean acidification using the MPWP as an analogue for future change. To assess this, we examined the extent to which molluscan diversity and community structure shift across the MPWP as recorded in the Sunken Meadow, Rushmere, and Moore House Members of the Yorktown Formation in southeastern Virginia.

Bulk samples, targeting each of the three members, were obtained from both museum and field sampling focusing on sites in southeastern Virginia. Molluscan specimens were sieved, sorted, identified, and counted. Descriptions and monographs were used to identify specimens to the species level when possible. Other supplementary resources included the Neogene Atlas of Ancient Life and World Register of Marine Species. Bivalves were counted if they had an identifiable hinge, and the number of hinged individuals was divided by two to estimate the minimum number of individuals. Gastropods were counted by apex or aperture, and the highest number of the two was recorded as the minimum number of individuals. Non-metric multidimensional scaling, rarefaction analyses, and multiple indices (Shannon-Weiner, Chao-1, etc.) were used to assess aspects of taxonomic richness, evenness, and community structure.

Across the MPWP, evenness increased at both the species and genus levels, whereas richness only increased at the genus level. These preliminary findings raise the possibility that rare species may have increased in abundance, while common species may have decreased in abundance across the MPWP. Studying how molluscan diversity changes across warming intervals in the fossil record contributes to our understanding of how modern mollusks will respond to global warming in the coming century.