IMPLICATIONS OF BETA: DIVERSITY PARTITIONING DURING MIOCENE-PLIOCENE BACKGROUND EXTINCTION

Previous work suggests that the structure of ecological diversity may vary across a major extinction boundary. This study uses additive diversity partitioning (ADP) to examine whether diversity partitioning varies during periods of background extinction. Patterns in beta diversity were analyzed at a variety of spatial scales within shallow benthic marine bivalve paleocommunities in the Miocene Eastover Formation and the Pliocene Yorktown Formation of the Virginia Coastal Plain. These stratigraphic units represent deposition in similar shallow subtidal environments. Sixteen bulk samples were collected from two locations along the James River in Surry County, Virginia using a hierarchical methodology involving multiple samples from each formation at individual cliffs, and multiple cliffs per location. Samples were wet-sieved to 2 mm. All bivalve material was sorted and counted at the species level. ADP analysis indicates that there is no change in overall diversity structure of the bivalve fauna during the 4.3 Ma represented by these units. When considering all collected data, there was no significant change from the Eastover to the Yorktown in either the raw beta values or the partitioning of beta diversity. This suggests that taxonomic rates of origination and immigration in the study area are similar to those of extinction and emigration. If taxa unique to the Yorktown in this data set are excluded, there is an overall decrease in beta diversity at all levels, but partitioning does not change. The lack of change in partitioning for both sets of data suggest that background extinction during this interval is not selective at any of the geographic scales included in the sampling hierarchy.