BORING AND ENCRUSTING ORGANISMS ON SPECIES OF PYCNODONTE (OSTREINA, GRYPHAEIDAE) THROUGH THE CRETACEOUS-PALEOGENE BOUNDARY IN PATAGONIA, ARGENTINA

Changes in the boring and encrusting communities during the Cretaceous-Paleogene transition in northern Patagonia were studied on shells of three species of Pycnodonte: P. vesicularis (Lamarck) is recorded in late Maastrichtian mudstones, P. burckhardti (Böhm) is restricted to similar facies of the early Danian, and P. sarmientoi Casadío to the late Danian. All three species were free lying types inhabiting muddy bottoms on a shallow shelf. Shells of these species were important in an ecosystem largely devoid of hard substrates. Therefore, they provided a necessary and scarce resource for the associated boring and encrusting organisms. P. vesicularis shows bioerosions referable to the ichnogenera Entobia, Caulostrepsis, Maeandropolydora, Rogerella, and Feldmannia?, while encrusting organisms are represented by juvenile oysters and the bryozoans “Reptocavea” excavata Canu and “Membranipora” minuscula Canu. In addition to the juvenile oysters, shells of P. burckhardti carried Entobia, Caulostrepsis, Maeandropolydora, Rogerella, Feldmannia?, Talpina, Oichnus, and the encrusting bryozoans “Membranipora” trimorpha Canu, “Diastopora” littoralis d’Orbigny, “Coscinopleura” planulata Canu, “Reptocavea” discoidea Canu, and “Euritina” lata Canu. P. sarmientoi has Entobia, Gatrochaenolites, Rogerella, Leptichnus, Pennatichnus, Maeandropolydora, Caulostrepsis, Talpina, Clionolithes, and fungal borings; encrusters included juvenile oysters, serpulids, balanids and the bryozoans “Reptotubigera” compacta Canu, “Reptotubigera” scalaris Canu, “Ballantiostoma” fibrosum Canu, “Reptocavea” discoidea Canu, “Membraniporella” capitata Canu, “Proboscina” striata Canu, “Hippoporina” planulata Canu, “Haplocheilina” spectabilis Canu, “Porella” levinseni Canu, and “Exochella” scalaris Canu. Results suggest that communities dependent on oysters as substrates underwent no significant changes in structure across the Cretaceous-Paleogene boundary. However, an important increase in diversity is recorded in the late Danian.