MORPHOLOGICAL VARIABILITY AND SURVIVORSHIP IN VENEROID BIVALVES DURING THE PLIO-PLEISTOCENE EXTINCTION IN FLORIDA

Recent studies of extinction in the fossil record indicate that morphological selectivity can drive evolutionary trends. These investigations have focused primarily on specific morphological characteristics of organisms, and few studies have directly analyzed the role of morphological variability in promoting survivorship. The purpose of this study is to examine the relationship between morphological variability and survivorship in veneroid bivalves during the Plio-Pleistocene extinction in Florida. The Plio-Pleistocene extinction (1.8 Ma) occurred in response to global cooling and changes in ocean circulation possibly related to the emergence of the Isthmus of Panama. Stratigraphic occurrences of veneroid bivalves were compiled from the literature to determine victim and survivor species. Fifteen veneroid genera were selected for study, each containing at least one extant species and one species that became extinct during the event. More than 1000 specimens representing 41 localities were examined from collections at the Florida Museum of Natural History and the Virginia Museum of Natural History. These specimens were sampled from the Pinecrest Beds, the Caloosahatchee Formation, and the Jackson Bluff Formation in Florida. Specimens were identified to the species level and digitally imaged. Landmark coordinate points for morphometric analysis were collected from the hinge, the adductor muscles, the pallial line, and along the shell outline at points representing maximum length and height. Generalized Procrustes techniques were applied to quantify morphological variability. The relationship between survivorship and variability was assessed using both parametric and non-parametric statistical tests. Preliminary results suggest that extinct and extant species do differ according to morphological variability.