THE TWO FOSSIL RECORDS: A PILOT STUDY IN THE FLORIDA PLEISTOCENE

The southeastern United States, particularly the state of Florida, possess a rich Cenozoic fossil record with abundant and well-preserved shelly fauna. The Cenozoic fossil record, which is dominated by unlithified sediments, has been typically examined by either ex situ field collecting or by acquiring a stratigraphically arranged series of bulk sediment samples. However, it is possible that such methods of collection may not represent the fossil record in its entirety. We posit a hypothesis that sediments enclosed inside large mollusk shells and adjacent matrix sediments represent two distinct fossil records that may differ in taphonomic history and provenance. To test this hypothesis, we compared fossil samples enclosed within large articulated shells of bivalve mollusks to fossil samples collected from sediments surrounding those shells.

We conducted our field sampling at the Florida Shell and Fill Quarry in Charlotte County, which encompasses geologic units ranging from the upper Pliocene Ochopee Member of the Tamiami Formation to the upper Pleistocene Fort Thompson Formation. These units represent a substantial interval of geological time (~ three million years). Following field collection, the shell samples and matrix samples were screen-washed, subsampled, and picked for gastropods and bivalves. Specimens were identified to species level whenever possible. Because samples obtained from large mollusk shells are dominated by small specimens (<2mm in maximum dimension), only 1-2 mm size fractions were compared to evaluate for differences and similarities between the two types of samples. Our preliminary results based on multiple pairs of shell and matrix samples suggest that the two types of samples differ in terms of diversity and faunal composition likely reflecting the different taphonomic histories of shell-enclosed and matrix-enclosed mollusks. These pilot results suggest that simultaneous sampling of matrix-enclosed and shell-enclosed fossils may result in a more comprehensive documentation of the fossil record. In addition, differences in the taphonomy and provenance of shell and matrix samples can potentially shed light on the taphonomic history and provenance of highly fossiliferous units that are observed frequently in the Cenozoic fossil record.