PHYLOGENETIC RELATIONSHIPS AND MORPHOLOGICAL CHANGE IN VENERICARDIA (BIVALVIA: CARDITIDAE) ON THE U.S. GULF COASTAL PLAIN DURING THE PALEOGENE

The world's climate is currently shifting at an unprecedented rate and we are still learning how this change will affect the evolution and ecology of modern organisms. The fossil record provides us with an opportunity to quantify biotic response to climate change over long timescales. The bivalve genus Venericardia, which is abundant and exceptionally well-preserved across multiple episodes of Paleogene climate change in the Gulf Coastal Plain, represents an ideal study system and may prove to be a useful proxy for modern climate change.

The primary goal of this research was to quantitatively reconstruct the phylogenetic relationships among venericard species and to explore the role that climate change may play in venericard evolution. This phylogeny allowed us to identify the major clades of venericards that occur in Paleogene units across the Gulf Coastal Plain, as well as to evaluate their relationships to European venericards. We used the phylogenetic framework to investigate the evolution of particular traits, including planicostate ornament. We then mapped morphometric data onto the phylogeny in order to study changes in shape across lineages. Finally, we compared the timing of phylogenetic and morphometric shifts to episodes of climate change.

Specimens of Paleogene venericards were examined from North American and European field and museum collections. The resulting database included more than 80 shell characters spanning more than 25 venericard species, with four outgroup taxa. Characters incorporated all observable morphological differences among species, including variation in ornament, hinge teeth, muscle scars, shell shape, and shell size. The majority of characters were unordered, multi-state characters and phylogenetic reconstruction was accomplished using a parsimony-based approach. Morphometric data were collected from digital images of the lateral and cross-sectional views of right valves. Morphology was quantified using 13 landmarks, which were then submitted to Procrustes superimposition and principle component analysis (PCA) to construct a morphospace. Preliminary phylogenetic results suggest that planicostate venericards may represent a monophyletic group.-8-2008-->