This paper presents evidence that fossil Crassostrea oysters experienced much greater longevity than their modern-day descendants. For extant animals, multiple studies have revealed a positive correlation between longevity and adult body size: the larger the organism's size as an adult, the longer its lifespan. Studies have also demonstrated that organisms that take longer to reach skeletal and/or sexual maturity also tend to have longer lifespans. Fossil Crassostrea oysters are generally larger than their modern-day counterparts, and ontogenetic growth curves suggest longer lifespans and generally longer growth intervals. This evidence should be of great interest to biblical creationists in light of the Bible's claim that humans in the pre-Flood and immediate post-Flood worlds experienced much greater longevities than modern-day humans. Animals may also have once experienced greater past longevity, since whatever genetic or environmental factors were enabling extreme human longevity were likely also operating across the animal kingdom.