EFFECTS OF PALEOGENE CLIMATE CHANGE ON MORPHOSPACE AND MORPHOLOGICAL DISPARITY OF VENERICARD BIVALVES

The Paleogene record of the U.S. Atlantic and Gulf Coastal Plain provides an ideal opportunity to examine the effects of climate change on the morphology and morphological diversity of marine invertebrates. The response of marine invertebrates to global warming and cooling is a complicated mosaic of extinction, migration, and evolutionary adaptation—the last of which has received surprisingly little attention in fossil studies, despite the opportunity provided to parse out the importance of these various factors.

In this study, we aim to quantify the morphological response of a monophyletic clade of marine bivalves, venericards (Family Carditidae), to intervals of Paleogene warming and cooling along the U.S. Coastal Plain. Data on over 800 venericard specimens, representing 32 species, were compiled from museum collections and span the early Paleocene to the Miocene from New Jersey to Texas. Morphology was quantified using nine homologous landmarks in lateral orientation and five landmarks in cross-sectional orientation. Landmark coordinates were transformed into Procrustes coordinates to remove variation due to size and rotation, then coordinates were converted into a morphospace using principal components analysis. Morphological disparity across the entire clade was calculated as the sum of variances with 95% confidence intervals derived from bootstrapping. Changes in morphology and morphological diversity were explicitly assessed across the Paleocene-Eocene Thermal Maximum (PETM), the early Eocene Climatic Optimum (EECO), and the Eocene-Oligocene cooling event (E/O).

Preliminary results indicate that venericard shape did not change significantly across the PETM or EECO, but venericards became smaller and more globose in shape with more compact hinges and wider-set adductor muscles after the E/O, in large part due to the extinction of planicostate forms at this time. Morphological diversity appears to decline across both the PETM and E/O, although this shift is only statistically significant for the latter. While taxonomic diversity of venericards peaks in the mid Eocene, morphological diversity peaks twice—once in the mid to late Eocene and another in the Oligocene.