PLIO-PLEISTOCENE EXTINCTION SELECTIVITY IN VENEROID BIVALVES: AN ANALYSIS OF MORPHOLOGICAL VARIABILITY AND SURVIVORSHIP

Although morphological selectivity has been demonstrated to drive evolutionary trends and influence recovery from mass extinctions, the link between morphological variability and survivorship is poorly understood. This study examines morphological variability and size selectivity in veneroid bivalves during the Plio-Pleistocene extinction in Florida, USA. The Plio-Pleistocene extinction (1.8 Ma) resulted in the extinction of ~65% of bivalves in the Caribbean and Western Atlantic. Species were designated as victims and survivors based on stratigraphic occurrences compiled from the literature. Fourteen pairs of closely related species were selected for analysis, each pair including one survivor species and one species that became extinct during this interval. More than 1500 specimens were examined from collections at the Florida and Virginia Museums of Natural History, representing 27 localities from the Caloosahatchee, Jackson Bluff, and Nashua Formations and the Pinecrest Beds in Florida. Specimens were digitally imaged., and landmark coordinate points were collected from the hinge, adductor muscles, pallial line, and points representing maximum length and height. Generalized Procrustes Analysis was applied to superimpose landmark configurations, and Procrustes Root Mean Square was calculated as an overall measure of multivariate morphological variability. Centroid size was computed to quantify body size. Differences in morphological variability and body size between victim-survivor pairs were assessed using parametric and non-parametric statistical tests. Results indicate that survivors were significantly more variable than victims. The variability difference persists, but is no longer significant when comparisons are controlled by locality. This result may be a consequence of reduced statistically power, or possibly indicate a role for ecophenotypy in accounting for variability differences. In contrast to morphological variability, body size does not appear to significantly affect survivorship in veneroid bivalves during this event. These results suggest that morphological variability may be a significant factor in promoting the survivorship of veneroid species during the Plio-Pleistocene extinction.