WHY ARE THE MOLLUSKS FROM THE EOCENE GOSPORT SAND SO BIG?

The late-middle Eocene Gosport Sand of the US Gulf Coastal Plain contains a dense fossil accumulation where the mollusks are unusually large in comparison to typical Paleogene Coastal Plain fossils. Both the underlying Lisbon and the overlying Moodys Branch Formations are also fossiliferous shelf sands, containing well-preserved mollusks, but individuals appear to be uniformly smaller in both of these units. Despite much paleoecological attention, no studies have specifically addressed this size difference or its cause. Here, we ask the question, are Gosport Sand mollusks truly larger on average than those in adjacent units, and if so, what might this be due to? We anticipate that the size difference will be statistically significant, and hypothesize that it is the result of higher primary production during deposition of the Gosport Sand. The hypothesis is tested using a combination of sclerochronologic and stable isotopic techniques.

Paleoproductivity has proven to be a particularly difficult variable to assess directly and with confidence in the sedimentary record. Instead, we take an indirect approach to the problem by asking what the predicted effects of increased production should be on the molluscan fauna. All else being equal, rapid growth is an expected ecophenotypic response to higher food availability. Therefore, if Gosport Sand mollusks exhibit more rapid growth and larger sizes than do their congeners in adjacent units, higher production can be inferred. Differences in longevity are more difficult to explain, but may also relate to paleoproduction through changes in population dynamics.

We compare the size distributions of 4 consistently-present taxa (the gastropods Agaronia and Natica and the bivalves Venericardia and Nucula) within bulk samples from field and museum collections from each of the three units, and examine their life histories to reveal size-age relationships in each unit. Stable isotope analyses of microsamples collected along the ontogenetic trajectory reveal seasonal temperature changes and hence provide a chronometer for growth. Once the best-fit size-age relationship is determined for a taxon using multiple specimens, the ages of additional individuals can be determined simply by measurement and application of the appropriate standardized growth curve.