Bioerosion structures in Quaternary marine mollusks from Argentina

Field and laboratory observations of individual shells of 40 dominant taxa (19 gastropods, 21 bivalves) identifi ed from bulk samples collected in 18 localities along the Atlantic Argentine coast (Río de La Plata-southern Santa Cruz, Patagonia), allowed the recognition of 12 ichnotaxa and durophagous scars. All modes of bioerosion were considered: internal and external at all scales (macro and microborings). The bioerosive structures (Caulostrepsis, Centrichnus, Entobia, Gastrochaenolites, Iramena, Leptichnus, Maeandropolydora, Oichnus, Pennatichnus, Pinaceocladichnus, Renichnus, Umbichnus) can be attributed to various activities produced by bivalves, gastropods, annelids, sponges, bryozoans and cirripedians. They belong to three ethological categories: Domichnia (dwelling), Fixichnia (anchoring) and Praedichnia (predation). The only bioturbation structure identified is Ophiomorpha nodosa. Overall, they document rich benthic littoral original palaeocommunities in the Bonaerensian and Patagonian areas (Southwestern Atlantic, SWA) and indirectly the first fossil record of Ctenostomate bryozoans for the marine Quaternary in Argentina. In gastropod shells, Domichnia structures are dominant (e.g., Entobia, Maeandropolydora). Bivalves exhibit predominantly Praedichnia traces (e.g., Oichnus). The Holocene deposits, where all the identified structures occur, show the highest ichnodiversity. The large macroscale approach in space and time of our study allowed the assessment of a latitudinal ichnodiversity pattern in agreement with that observed for benthic molluskan taxa in response to changes in sea surface temperature conditions of the Mar Argentino. In Patagonia, the ecological interactions remained similar and stable within the original invertebrate palaeocommunities across time, at least since ca 400 Ka (MIS11). The dominant ichnogenus recognized is Entobia (60%), followed by Oichnus (50%); Maeandropolydora (47.5%) and Leptichnus (35%).