Three species of oysters from the middle Eocene Cubitostrea lineage are used for local and regional correlation and zonation of sediments in the Gulf coastal plain. However, oysters from the middle Eocene Piney Point Formation in Virginia also include the co-occurrence of specimens that exhibit the morphologic characteristics of these 3 Gulf Coast species.
Examination of approximately 150 specimens from 8 bulk samples collected in a vertical sequence along the Pamunkey River revealed that, in addition to co-occurring, a continium exists from one form to the next. The general shape of the forms is related to size, with juvenile oysters characterized by a triangular shape and prominent ribs, whereas the adults are oval with large auricles and a saddle-shaped form.
Results indicate that the morphologic variation of oysters in the middle Eocene sediments of Virginia does not represent an evolutionary lineage with each form a species, as proposed for the Gulf coastal plain. Rather, the forms are related to the growth stages of the species C. sellaeformis. Therefore, the Gulf model is not applicable to the middle Eocene sediments of Virginia and possibly not to other depositional areas.
Oysters from the underlying Nanjemoy Formation (lower Eocene) previously included within the species C. sellaeformis should not be so designated. This distinction is based on the coarser and fewer number of ribs on the Nanjemoy oyster as compared to C. sellaeformis from the Piney Point Formation.