Previous study of drilling predation on the bivalve Chione during the late Neogene of Florida suggested that prey-size selectivity of predators was disrupted by species turnover and morphological change within the prey genus. More recent experimental work, however, showed that at least some of these changes can be attributed to the confounding effects of facies shifts between naticid-dominated, muricid-dominated, and mixed predator assemblages. As muricids have the most abundant and continuous fossil record and are most responsible for predation on the Chione bivalve in modern benthic ecosystems of Florida, we use new criteria to isolate the muricid component of the Chione drillhole record and analyze the history of this type of predator independently. Our analysis, based on drilled Chione from four Plio-Pleistocene formations in Florida, does not support the previous scenario of disruption at the end of the Pliocene followed by predator recovery. Rather, selected prey size has steadily increased since the middle Pliocene, although the stereotypy of prey-size selection behaviors has decreased. In order to explain this trend, I performed a series of statistical analyses to explore factors most likely to have influenced muricid prey-size stereotypy. The timing of Species turnover within the prey lineage or change in prey phenotype does not correlate with the timing of changes in prey-size stereotypy and, therefore, cannot explain the observed changes in muricid behavior. Presence of secondary predators may also influence predator-prey interactions, because predators forage sub-optimally to ensure greater safety in the presence of enemies. Results indicate that secondary predation pressure decreased at the Caloosahatchee-Bermont boundary without any evident change in muricid prey-size stereotypy and hence refute the hypothesis that secondary predation induced sub-optimal foraging. A third factor tested is prey density, which plays a major role in predator-prey interactions in other systems by thwarting a predator's ability to single out the preferred individual prey. Increased Chione prey density correlates with and provides support for increased confusion among the muricid predators and hence driving the increased sub-optimal behavior reflected by the increased variability in prey-size selection. This is the first time prey density effect has been considered and its importance here over all other factors suggests that it may be a critical factor in short- and long-term predator behavior trends in fossil record.

The ecological impact of past extinction events is one of the central issues in paleobiology. In face of present environmental changes, a better understanding of past extinctions will enable us to identify the magnitude of biodiversity crises and their underlying processes. The Late Neogene was a time of extraordinary climatic reorganization, including Northern Hemisphere glaciation, the rise of the Central American Isthmus, and associated changes in environmental conditions. Therefore, the Late Neogene extinctions of marine molluscs of South Florida present an ideal platform to examine the interaction between environmental changes and biotic response. In the present study, three different aspects of the Late Neogene extinctions are examined: temporal diversity patterns, selectivity patterns, and the impacts of these extinction events on ecological interactions. In the first part of this study, the diversity pattern of marine bivalves of Florida during the Late Neogene. Using bulk samples enables to take account of varying sampling intensity and underlying relative abundance distributions in diversity estimation. Comparison of sample-standardized diversity analyses shows that both richness and evenness of marine bivalve community declined at the Tamiami - Caloosahatchee transition, which coincides with the proposed first phase of the Late Neogene extinctions at the end of the Pliocene. Although magnitude of biodiversity loss was severe during these late Neogene extinction events, extinction risk was non-randomly distributed across taxa. Selectivity analyses, a combination of both commonly used non-parametric tests and logistic regressions, suggest that abundance or local population size was positively related with survivorship during the late Neogene. As other biological or ecological traits can influence this observed relation between abundance and extinction vulnerability, multivariate approach is used to control for these traits. Even after effects of geographic range and feeding mode is considered, the positive relation between abundance and survivorship, which supports predictions from biological studies, is evident in case of these Late Neogene extinction events. While present analyses show that the increase in relative abundance of Chione is a major factor in driving changes in community compositions, interactions between Chione and its' drilling predators also varied during the Late Neogene. This study suggests that identification of predators is a critical part of evaluation of prey-predator interactions. When drill hole traces of two predatory gastropod groups, muricids and naticids, are differentiated based on a revised site selectivity criteria, temporal trend of prey size selectivity differs from previous reports. Both groups exhibit some changes in predatory behavior during phases of the Late Neogene extinctions, suggesting that previous hypothesis of prey turnover at the Caloosahatchee - Bermont transition cannot explain the observed temporal trends of prey size selectivity in the present study.