The upper middle Eocene Gosport Sand of the U.S. Gulf Coastal Plain preserves a spectacularly dense accumulation of fossil mollusks where the shells appear unusually large in comparison to congeners in adjacent units. This apparent size difference has not been addressed in paleoecological work to date. We use size frequency distributions of taxa drawn from bulk samples to demonstrate that individuals from the underlying Lisbon and the overlying Moodys Branch formations, both also fossiliferous shelf sands containing well-preserved mollusks, are uniformly smaller for four of the five taxa examined thus far. This difference does not appear to be related to winnowing or size sorting in the slightly coarser-grained Gosport, nor to differences in sample size between the three units.

The larger Gosport mollusks must therefore be due to faster growth and/or longer lifespan. To assess life history trajectories and size-age relationships, we used sclerochronology and stable isotope geochemistry on three taxa common to the Gosport and neighboring formations. Stable isotope analyses of microsamples collected along the ontogenetic trajectory reveal seasonal temperature changes and hence provide a chronometer for growth. Data for the gastropod Agaronia indicate that Gosport individuals lived less than two years but attained sizes four to five times larger than those in the Lisbon and Moodys Branch formations. Specimens of the bivalve Venericardia lived an average of six years, but those in the Lisbon and Moodys Branch formations are only half as large as of those in the Gosport. Isotope data and growth banding from the bivalve Nucula suggest much slower growth in individuals from the Lisbon and Moodys Branch formations. We speculate that larger size and faster growth is the result of higher primary production during deposition of the Gosport Sand. Rapid growth is an expected ecophenotypic response to higher food availability.