EXPLORING THE LINK BETWEEN RARITY AND MOLLUSCAN EXTINCTION IN THE CENOZOIC RECORD OF THE U.S. COASTAL PLAIN

A handful of studies have investigated the influence of rarity on the survivorship of fossil taxa, using a variety of strategies from correlating stratigraphic duration with abundance, to assessing shifts in evenness across extinction events. In this study, we apply a novel approach to testing the link between rarity and extinction, by expanding the definition of rarity to include aspects of both spatial and temporal variability in abundance and by controlling for phylogenetic bias. We also compare patterns of selectivity across multiple extinction events that differ drastically according to magnitude, duration, and causal mechanism.

Data on fossil abundance (including over 100,000 specimens, representing over 300 species, across 24 localities) were compiled from our field studies of Neogene mollusks along the mid-Atlantic Coastal Plain. These data were supplemented with two case studies from the molluscan Coastal Plain literature, extending across both the end-Cretaceous (over 100,000 specimens, representing 121 subgenera, across 256 localities) and end-Eocene extinctions (27,000 specimens representing 120 subgenera, across 20 localities). Survivorship of molluscan species was assessed during both background and mass extinction intervals, using occurrence data compiled from the Paleobiology Database and the literature. Proxies of survivorship, including stratigraphic duration, were then correlated with several metrics of rarity, defined here to include mean abundance, spatial and temporal variability in abundance, number of occurrences, and habitat specificity. Comparison of sister species within closely related genera suggests that phylogenetic control does not significantly influence these patterns of extinction selectivity. Although our results varied considerably among study sites and time intervals, little evidence was found to suggest that abundance promotes survivorship in Coastal Plain mollusks during the Cenozoic. The possibility that taphonomic processes are removing extremely rare species from the fossil record and blurring possible links between rarity and extinction requires further examination.