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

The effect of decreased morphological variability increasing extinction risk has been previously documented (Kolbe et al., 2011), but the effects of extinction on the morphology of surviving species is less well known. Here we investigated changes in the morphology of Cyclocardia granulata across the Plio-Pleistocene extinction in Virginia and North Carolina. 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 morphological variability changes may allow us to predict which species may be at risk for extinction in the future.

We chose the bivalve Cyclocardia granulata because it is present in the pre-extinction Yorktown Formation and the post-extinction Chowan River Formation. We built a dataset of 3-dimensional models of 84 specimens using Structure from Motion. In 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, a Principle Components Analysis (PCA), and a Canonical Variate Analysis (CVA). In addition, we compared bootstrapped variance across each PCA axis to see how shape variation was altered by the extinction event.

Morphological differences between pre- and post-extinction specimens are present along axes one and four in the PCA. Positive values of axis one represent more rounded outlines of the shells while negative represent more triangular outlines. Positive values of axis four are more globose and negative values are less globose with steeper slopes. Pre-extinction shells are more rounded in outline and are less globose or inflated in profile. This pattern is robust and remains when random shells are removed from analysis. These results are also seen with the CVA. The variance tests illustrate that there is less variation in shell shape after the extinction event, but these results are not statistically significant. These differences potentially illustrate changes due to selection pressures likely from the lowered upwelling rates and cooler temperatures. It is possible that the selection pressure is also related to predation since the presence of predatory drill holes decrease in frequency after the extinction.