LIFE HISTORY AND PALEOENVIRONMENT OF THE ICONIC MIDDLE EOCENE OYSTER CUBITOSTREA FROM STABLE ISOTOPE ANALYSIS

Cubitostrea sellaeformis is a large, saddle-shaped ostreid iconic of the middle Eocene Lisbon Formation (Claiborne Group) of the US Gulf Coastal Plain. The Lisbon Fm hosts a rich molluscan assemblage in which stenohaline, and deeper-water taxa are especially diverse and small solitary corals are common. The fauna, combined with a sandy marl lithology, suggests a fully marine depositional environment and an offshore setting. Oysters today, however, inhabit nearshore brackish conditions, suggesting that the modern representatives may not serve as adequate analogs for Cubitostrea in the past. Cubitostrea grows to quite a large size (commonly up to 15 cm), and shells can be 2+ cm thick, suggesting they could be slow-growing, longer-lived animals, a life history more consistent with stable, fully marine conditions. As oysters grow by accretion, we use stable isotope sclerochronology to determine whether data are more consistent with marine or brackish conditions. The shells reveal growth bands in cross section that correspond to prominent ‘shelves’ or growth breaks on the outer shell surface. Shells often exhibit at least 20 major growth breaks, consistent with a potentially long lifespan. Stable oxygen isotope data from samples collected along the growth axis reveal approximately sinusoidal variation that is more likely driven by temperature than by salinity. Cubitostrea isotope values are generally comparable to those from aragonitic bivalves, gastropods, and fish otoliths from the Lisbon formation after the difference in mineralogy is corrected for. Similarity with values from coeval fully marine taxa suggests that C. sellaeformis also inhabited a marine environment. Additionally, growth breaks correlate with negative isotope values, indicating reduced growth during summers. C. sellaeformis is also the culmination of a postulated ancestor-descendant series in which ancestors are much smaller and resemble the juveniles of C. sellaeformis, suggesting heterochronic evolution via peramorphosis. Isotope data on ancestor and descendant will provide constraints on lifespan to determine whether hypermorphosis or acceleration is the more likely method. The longer lifespan associated with the former would again be more consistent with a stable, fully marine environment.