MORPHOLOGICAL VARIATION AMONG PLIO-PLEISTOCENE VENEROID BIVALVES: GEOGRAPHIC AND STRATIGRAPHIC DISSECTION

Analyses of morphological variability during radiations and mass extinctions are crucial to understanding these critical transitions. This study examines morphological variability among veneroid bivalve species from Florida during the Plio-Pleistocene extinction. An earlier phase of this work suggested that morphological variability promoted the survivorship of veneroids during the extinction. The new analyses evaluate the geographic and stratigraphic components of morphological variability within and among localities to determine their relative contributions to the overall variability of a given species, and their effects on morphological selectivity.

The Plio-Pleistocene extinction (1.8 Ma) resulted in the loss of 65% of bivalves in the Caribbean and western Atlantic. Fourteen pairs of closely related species were analyzed, each including one surviving species and one victim that became extinct during this interval. More than 1500 museum specimens of these species were examined, from 27 localities in the Caloosahatchee, Jackson Bluff, and Nashua Formations and the Pinecrest Beds. Specimens were digitally imaged, and coordinates were determined for landmark points. Procrustes Analysis was applied to calculate morphological variability, and differences between the victim and survivor in each pair were assessed statistically within and among localities. Earlier analyses indicated that survivors tended to exhibit greater morphological variability than survivors, but these results warranted further investigation because of variation in the number and location of samples from species to species. In the new analyses, assessments of geographic and stratigraphic patterns of morphology and morphological variability were conducted. Preliminary results suggest a range of morphological attributes for species with respect to variation among samples; in some cases, species exhibited systematic morphological variation from locality to locality, whereas for others, variation was essentially random. These patterns illustrate how veneroid morphology responds to geographic and stratigraphic gradients; if greater morphological variability corresponds to a species' presence in a greater range of locations or environments, this may explain how variability actually promotes survivorship.