A Revised History of Regional Mass Extinction in the Florida Neogene: New Data and Approaches

A wave of extinctions hit tropical and subtropical American marine communities during the Late Neogene in one of the most severe biotic crises in the last 65 Ma. The timing and magnitude of the extinctions, as well as the degree of recovery, however, remain controversial, especially in Florida. Although Florida has historically been one of the best-studied regions for Neogene fossils, previous studies did not employ a standardized sampling regime, which is necessary to account for sampling biases in richness estimates.

In this study, we analyzed bulk-sampled collections of over 15,000 Plio-Pleistocene gastropods on a bed-by-bed scale for four units: “middle” Pliocene Upper Tamiami (lower Pinecrest Beds), Late Pliocene Caloosahatchee, Early Pleistocene Bermont, and Late Pleistocene Fort Thompson formations. Results using a standardized sample size (n=125) show that individual beds within the Pliocene Tamiami and Caloosahatchee formations contain, on average, roughly 40 gastropod genera, whereas individual Pleistocene beds from the Bermont and Fort Thompson formations contain less than 20 gastropod genera at the same level of sampling effort. A similar timing for abrupt richness decline was found for preliminary analyses conducted at the species level for the same data set.

The results of this new analysis are noteworthy in two respects. First, we find that a single pulse of extinction occurred for gastropod genera at or near the Plio-Pleistocene boundary (1.8 Ma). This event is more than half a million years later than reported in studies based on non-standardized sampling at the species level, and the revised timing changes the scope of possible external triggers of the extinctions. Second, our analysis suggests that high extinction rates were not balanced by high origination rates, at least at the genus level; early and late Pleistocene faunas are, in fact, depauperate at the genus level when compared to the Pliocene.