BIOEROSION ON OYSTERS ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN ALABAMA AND MISSISSIPPI (USA)

Sclerobionts on oysters from the Late Cretaceous and Paleogene of southern Alabama and northern Mississippi show that boring clionaid sponges increased their abundance across the extinction boundary while other encrusters and borers were significantly reduced. The bioerosion pattern across the K/Pg boundary is distinctive on similar shelly substrates such as Exogyra costata and Pycnodonte convexa. Nine sclerobionts were found on these oysters: Entobia, Gastrochaenolites, Oichnus, and Talpina borings; serpulids; oysters; encrusting foraminiferans; Stomatopora and “Berenicea” bryozoans. Entobia became more common on these oyster substrates in the Danian (earliest Paleocene) than in the Maastrichtian (latest Cretaceous). Gastrochaenolites shows the opposite distribution. The encrusting serpulids are more common on the Maastrichtian oysters than on those from the Danian. No bryozoans, encrusting oysters or foraminiferans are found on Danian E. costata and P. convexa. An interpretation of these patterns is that clionaid sponges survived the extinction and flourished on the oyster shells. The other sclerobionts were greatly reduced and apparently suffered significant extinctions. This could be because the sponges were opportunistic and could survive on any available carbonate hard substrate while the others were more specialized.