MORPHOLOGICAL CHANGES IN CYCLOCARDIA GRANULATA FOLLOWING PLIO-PLEISTOCENE EXTINCTION IN VIRGINIA AND NORTH CAROLINA, WESTERN ATLANTIC

While many studies have investigated the taxonomic effects of extinction events, the effects of extinction on the morphology of surviving species is less well known. Changes in morphology may illustrate what occurred during the extinction and how organisms may change in the future. Here we investigated changes in the morphology of Cyclocardia granulata across the Plio-Pliestocene extinction in Virginia and North Carolina. During this event about two thirds of the bivalve fauna in the Western Atlantic went extinct. In Virginia and northern North Carolina species diversity continues to be low to this day. This is important because the Pliocene was the last time global pCO2 levels were as high as they are today (approximately 400 ppm). Determining how the morphology changed may allow us to predict what will occur in the future due to anthropogenically increased global temperatures and pCO2 levels.

We chose to investigate the morphology of Cyclocardia granulata because it is present before and after the extinction event and is abundant in the Pliocene and Pleistocene sediments of Virginia and North Carolina. Specimens previously collected from the Pliocene Yorktown Formation and Pleistocene Chowan River Formation were used for this project. We built 3D models of Cyclocardia granulata shells using Structure from Motion. With this technique, 2D photographs taken around the shell are used to reconstruct the 3D shape. We then used the R package ‘geomorph’ to select semi-landmarks on the surface of the shell and performed a Procrustes transformation and a Principle Components Analysis (PCA).

Initial results of the PCA suggest that Cyclocardia granulata specimens pre- and post- extinction differ along axis 1. Pre-extinction shells are more rounded in outline than the post-extinction specimens. The results also suggest that along axis 4 pre-extinction shells are less globose in profile with more angular slopes. These differences potentially illustrate changes due to selection pressures likely from the lowered upwelling rates and cooler temperatures.