THE RECORD OF SCLEROBIOTIC COMMUNITIES CHANGES THROUGH THE CRETACEOUS-PALEOGENE TRANSITION IN PATAGONIA, SOUTHERN ARGENTINA

Oysters are among the more characteristic fossils of the marine rocks of the Cretaceous-Paleogene transition in Patagonia (Argentina). They played an important role in the establishment of diverse sclerobiotic communities, including fungi, algae, sponges, arthropods, bryozoans, phoronids, polychaetes and mollusks that used their shells as a substrate. Changes in sclerobiotic communities through the Cretaceous-Paleogene boundary were recorded in Patagonia on eight oyster species. Pycnodonte vesicularis and Amphidonte mendozana from the late Maastrichtian were encrusted by 4 bryozoans: Poricella tripora, Flustrellaria sp., Akatopora sp. and ?Trichinopolia sp. The oysters Gryphaeostrea callophylla, P. burckhardti and Turkostrea argentina from the lower Danian were colonized by 9 encrusting bryozoan species: Balantiostoma sp., Plagioecia sp., ‘Berenicea’ sp., Aspidostoma onychocelliferum, Mesenteripora sp., Tricephalopora sp., Conopeum ?okaiana, Heteroconopeum arborea and ?Balantiostoma sp. Oysters from the late Danian (i.e. Ostrea wilckensi, P. sarmientoi and Cubitostrea ameghinoi) supported the highest diversity (24 species) of encrusting bryozoans, including: Poricella tripora. Poricella sp., Balantiostoma elongata, B. spectabilis, Balantiostoma sp., Platonea compacta, Tremogasterina problematica, Proboscina cf. striata, Pyriporella ameghinoi, Pyriporella sp., Plagioecia sp., Plagioecia aff. cristata, Disporella discoidea, Disporella sp., ‘Berenicea’ sp., Aspidostoma sp., Mesenteripora sp., Voigtopora sp., Monoporella convexa, ?Tricephalopora sp., Actinopora robertsoniana, Oncousoecia sp., Electra sp. and Ceriopora sp. Most of the oysters contained borings assigned to the ichnogenera Entobia, Maeandropolydora, Gastrochaenolites, Rogerella, Oichnus, Talpina, Clionolithes and Pennatichnus. Results show that the sclerobiotic communities associated with the oysters show no major changes across the Maastrichtian-Danian boundary. However, an important increase in the diversity of bryozoans is recorded during the Danian in Patagonia. This change after the earliest Danian correlates with an increase in the number of species of corals, mollusks, echinoids and crabs derived from low latitudes, reflecting higher seawater temperatures spreading south.