Many Cenozoic species of Crassostrea lived in fully marine environments where they secreted large and thick shells. In contrast, living Crassostrea are restricted to brackish, hypersaline, and intertidal environments where they secrete comparatively smaller and thinner shells. In order to understand the reasons for these differences, I examined the ontogeny and paleoecology of fossil and living Crassostrea. d18O and d13C profiles of late Oligocene Crassostrea gigantissima and Quaternary Crassostrea virginica shells show that skeletal growth increments in the ligamental area formed annually from seasonally varying growth rates. Life spans and growth rates estimated from similar growth increments in Miocene thick-shelled Crassostrea titan and Quaternary thin-shelled C. virginica show that C. titan lived two to three times longer and grew faster than C. virginica. Crassostrea titan valves are thicker than C. virginica valves at all ages, suggesting that faster growth rates allowed C. titan to reach a size refuge earlier in life. A high percentage of failed drilling attempts in young C. titan indicates that their thick shells successfully deterred muricid predation. In contrast to C. virginica, C. titan and gigantissima shells are mostly composed of chalky deposits. Results show that C. virginica calcified their shells close to isotopic equilibrium with respect to their environment. Skeletal growth breaks allow estimation of d18Owater and paleotemperature from fossil oysters.

These results are consistent with the hypothesis that fully marine C. titan and gigantissima rapidly grew thicker shells in order to deter increased exposure to stenohaline predation. Association between these oysters and phosphatic sediments, intense bioerosion, and other large suspension feeders suggest that formation of thicker shells was predicated on a high or prolonged planktonic food supply resulting from nutrient-rich upwelling. The stratigraphic and biogeographic distributions of thick-shelled Crassostrea are consistent with the view that they evolved from euryhaline populations originally living in estuarine refuges. Both increased food availability and the ability to rapidly grow thicker shells may have permitted these oysters to leave estuarine refuges for fully marine environments multiple times in the Cenozoic. Their genetic differentiation from estuarine populations resulted in allopatric speciation and iterative evolution.