Tertiary Crassostrea oysters grew large and thick shells, whereas their descendants, living Crassostrea, grow comparatively smaller and thinner shells. To test for ecological differences between fossil and living Crassostrea that may account for differences in body size, the stable isotope sclerochronology was examined of two North American species, late Oligocene C. gigantissima (Belgrade Formation, North Carolina) and Quaternary C. virginica (Chesapeake Bay and Mississippi Delta), both purported to share an ancestor-descendent relationship. δ¹⁸O and δ¹³C profiles across skeletal growth increments in two well-preserved C. gigantissima shells show significant differences with profiles from Pleistocene and Recent C. virginica. Significantly higher δ¹⁸O and δ¹³C values and smaller seasonal isotopic ranges with less variability show that C. gigantissima lived in a more fully marine environment than C. virginica. Consequently, C. gigantissima probably was exposed to more predation, competition, and bioerosion than C. virginica. Isotopic profiles also show that C. gigantissima formed skeletal growth increments annually from seasonally varying growth rates, which permits estimation of life span and growth rate. Thicker shells and faster growth rates in C. gigantissima may reflect the greater exposure to fully marine predation and bioerosion, as well as to the conditions associated with higher salinity. Associations between C. gigantissima and phosphatic sediments, intense bioerosion, and other large suspension feeders in the Belgrade Formation are suggestive of an elevated planktonic food supply that may have permitted C. gigantissima to grow thicker shells. If estuaries and lagoons have served as refugia from marine predation since the Mesozoic, as these environments do today, then C. gigantissima may represent a lineage that had left its brackish refuge for shallow-marine environments in the Eocene. The replacement of C. gigantissima by C. virginica in the early Miocene may not represent an ancestor-descendent relationship, but an extinction of C. gigantissima and continued survival of thin-shelled Crassostrea in brackish environments.