Multiple paths to extinction: Testing ecological predictors of extinction risk using the marine bivalve fossil record

The geologic record provides a series of natural experiments which can be used to evaluate the long-term influence of species ecology on extinction. Ecological characteristics such as abundance and geographic range size are widely believed to influence extinction risk and to have influenced diversity dynamics of clades. While these effects have been empirically tested for many extant and extinct taxa, most studies have treated such ecological characteristics as having independent impacts on extinction rates. Here I use the Eocene marine fossil record of the U.S. Gulf Coastal Plain to assess the covariance between species ecological characteristics and their combined effects on extinction rates. Contrary to the general assumption of independence, preliminary results for a broad sampling of bivalve species show a positive covariance between body size and geographic range and between body size and abundance. Given the non-independence of abundance, body size, and geographic range at this broad taxonomic scale, I use path analysis to examine the direct and indirect effects of these ecological characteristics on extinction rates for a subset of bivalve superfamilies (Carditoidea, Pectinoidea, Veneroidea) that differ in basic aspects of life history and ecology. Using a model selection approach, I test the generality of extinction rate predictors among these three groups and find that geographic range explains much of the variation in extinction probability with body size having relatively little effect. The marine invertebrate fossil record offers rich opportunities for testing the interaction of factors hypothesized to determine origination and extinction rates.