The late–middle Eocene Gosport Sand of the United States Gulf Coastal Plain preserves a spectacularly dense accumulation of fossil mollusks in which shells appear unusually large in comparison to congeners in adjacent units. We use size-frequency distributions of common taxa drawn from bulk samples to quantify size differences between the Gosport Sand and the underlying Lisbon and overlying Moodys Branch Formations. In all but one of the twelve lineages examined, Gosport Sand individuals are larger than those in adjacent units. As taphonomic processes such as sorting or winnowing cannot account for observed differences, the larger body sizes in the Gosport Sand must have resulted from differences in life history. We used sclerochronology and stable isotope geochemistry to assess growth trajectories for twelve lineages common to the three formations. Isotope analyses of microsamples collected along the ontogenetic trajectory reveal seasonal temperature changes, providing a chronometer for growth. Size-age data indicate that, while life spans vary somewhat among the three units, Gosport Sand taxa almost universally grew faster than those in the two adjacent formations. While growth rate can be enhanced by several factors, we hypothesize that rates of primary production were higher during deposition of the Gosport Sand. Ba/Ca ratios along the growth trajectories of shell carbonate exhibit spikes in Gosport Sand mollusks that are indistinguishable from those observed in modern shells that accrete in high productivity settings.

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