CHANGES IN THE SCIAENIDAE DURING THE EOCENE/OLIGOCENE AS INDICATED BY OTOLITHS IN THE CENTRAL GULF COAST

Taxa of the family Sciaenidae (the drums and croakers) occur commonly in the marine Eocene and Oligocene otolith assemblages of the Gulf Coast Paleogene. Their abundance and other factors suggest the sciaenids as potential indicators of change across the marine Eocene-Oligocene transition in the central Gulf Coast. Although complete or partial skeletons of bony fishes are extremely rare in the Gulf Coast Paleogene, fish otoliths occur frequently in the marls, clays, and sands of the central Gulf Coast and are excellent proxies for the evolution of fish assemblages. Several other factors contribute to the usefulness of sciaenid otoliths in interpreting changes during the Eocene-Oligocene. Extant sciaenids are almost exclusively confined to shallow marine, coastal, and estuarine environments with a few freshwater forms. In the number of individuals or biomass in the modern Gulf of Mexico, sciaenids are among the top three. Furthermore, sciaenids appear to be restricted to continental margins and are absent from the Pacific Oceanic Island groups. Present-day distribution of sciaenids is predominantly tropical to subtropical and diminishing in more temperate zones. Open ocean regions and cold temperate zones appear to serve as effective barriers to sciaenid distribution. These parameters indicate that sciaenids should be accurate indices of temperature changes during the Eocene-Oligocene transition. To investigate this premise, sciaenid otoliths from several late Eocene sites (Yazoo Clay) in Louisiana were collected (bulk sampling and systematic surface collecting). The late Eocene sciaenids were compared to Oligocene sciaenids in similar paleoenvironments in Louisiana (Rosefield Marl Beds) and Mississippi (Byram and Mint Spring formations). Late Eocene sciaenid assemblages were significantly different from Oligocene sciaenid assemblages in both composition and quantity. The highly abundant late Eocene sciaenid “genus Sciaenidarum” claybornensis appears to end rather abruptly in the late Eocene and is unknown in the otolith assemblages of the Oligocene. Otoliths of Sciaena psuedoradians become one of the most common sciaenids in the Oligocene and show a substantial increase in size. Several new sciaenid forms, such as Aplodinotus, emerge in the Oligocene as the assemblage becomes more diversified.