RARITY AND EXTINCTION DURING BACKGROUND INTERVALS: ARE ECOLOGICAL PATTERNS PRESERVED IN THE FOSSIL RECORD?

Ecological studies suggest that rare taxa are more likely to go extinct than abundant ones, but the influence of abundance on survivorship in the fossil record has received little attention. A recent analysis of North American bivalve subgenera found no link between abundance and survivorship across the end-Cretaceous mass extinction, regardless of abundance metric, taxonomic rank, or spatial scale examined. In this study, we follow up on this general question by investigating the relationship between rarity and extinction during background extinction intervals in the fossil record.

The majority of the data used in this study was collected from the Sunken Meadow Member (early Pliocene) of the Yorktown Formation. Thirty bulk samples were obtained from three localities along the James River in southeastern Virginia. Samples were sieved and bivalve material greater than 1 mm was sorted and identified to species level. Three metrics (raw, rank, and proportional) were used to quantify abundance and data on life habits and shell mineralogy were compiled to evaluate the possibility of preservational bias. Geological durations (i.e., stratigraphic ranges) of species were collected from the literature and used as a proxy for survivorship throughout this interval of background extinction. The relationship between abundance and duration was assessed via both parametric and non-parametric statistical tests. Spatial and temporal variability in population size was also quantified and compared to duration. Preliminary results indicate that abundance and survivorship are not statistically significantly correlated to each other. Rare species do tend to have shorter geological durations, but only when the extremely abundant species are removed from the analysis. Additional literature data were compiled for other background intervals and taxa, to provide a comparison for the field-collected data.