DETERMINING THE EFFECTS OF THE MID-PLIOCENE WARM PERIOD ON THE MORPHOLOGY OF GLYCYMERIS SUBOVATA (BIVALVIA)

While the temperature and environment have been reconstructed across the mid-Pliocene Warm Period (MPWP), relatively few studies have examined the biotic response of marine invertebrates. The goal of this research is to examine changes in morphology of the bivalve Glycymeris subovata from the Virginia Coastal Plain across this warming interval using three-dimensional imaging. G. subovata is a shallow, infaunal suspension-feeding bivalve that has a high abundance and quality of preservation during the MPWP. With a potential for increased temperatures in the near future, it is important to understand the effects of warming on marine ecosystems.

We collected 212 Glycymeris subovata specimens from field localities and museum collections across the Virginia Coastal Plain. In total we sampled nine different localities with exposures of the Sunken Meadow, Rushmere, and Moore House Members of the Yorktown Formation. Only the largest specimens from each locality were targeted to avoid including juveniles. We collected three-dimensional morphometric information by creating digital models of each specimen using Structure from Motion. Forty images were taken in a circular pattern around each shell. Digital models were generated in Agisoft Metashape and scaled to actual size before calculating centroid size from 15,000 XYZ data points in R. Shape differences were quantified by first performing a Procrustes analysis to scale and align all models to uniform size and location in digital space. We then populated each model with 200 equally spaced XYZ data points in R and performed a principal component analysis to determine differences in shape.

Preliminary results indicate significant shifts in size and shell globosity, symmetry, and width before and throughout the MPWP. The period after the MPWP also records a significant size increase after warming, but no shape differences are observed. These results indicate that increased temperatures may have limited the growth rate and/or size of Glycymeris subovata.