Phylogenetic relationships and morphological changes in Venericardia on the Gulf Coastal Plain during the Paleogene

The bivalve genus Venericardia is abundant and exceptionally well-preserved on the U.S. Gulf Coastal Plain during the Paleogene. The climate was extremely variable during the Paleogene, allowing the system to serve as a proxy for modern climate change. The primary goal of this research was to quantitatively reconstruct the phylogenetic relationships among venericard species and to explore patterns in venericard morphology. A phylogeny was produced from 70 qualitative multi-state characters applied to over 37 species and analyzed using a parsimony-based approach. This phylogeny identified the major clades of venericards that occur in Paleogene units along the Gulf Coastal Plain, as well as their relationships to European venericards. The phylogenetic framework was applied to investigate the evolution of external ornamentation and the validity of proposed subgenera. Landmark data were collected via digital images of the internal and cross-sectional orientations of right venericard valves and used to explore the morphometric relationships among hypothesized groups. Finally, the timing of phylogenetic events and morphometric changes were compared with climate shifts. The phylogeny demonstrates that alticostate venericards form a monophyletic group within the venericard genus. None of the proposed venericard subgenera are monophyletic, although European and North American venericard species are closely related. The interior and cross-sectional morphospaces indicate that alticostate and planicostate venericards are morphometrically separate and that the proposed subgenera are all morphometrically distinct. Venericard diversity increased after the Paleocene-Eocene Thermal Maximum, although there is no significant morphological change across the climate shift. After the Eocene-Oligocene Transition, venericards suffered an extinction and became more globose in shape. These patterns could suggest that venericard morphology responds more strongly to temperature decreases than to increases