Boring polychaetes on oyster shells from the Upper Cretaceous of Patagonia: A possible case of commensalism?

BREZINA, S. ; CASADIO, SILVIO

Congreso; 18 International Sedimentological Congress; 2010

IAS

The oyster Pycnodonte (Phygraea) vesicularis (Lamarck) is recorded in the Maastrichtian mudstones of the Jaguel Formation. This species was free-lying and inhabited muddy bottoms in a shallow marine shelf. Therefore, oyster shells offered relatively stable habitats for the settlement of marine organisms/hard substrate communities. The most conspicuous  borings recorded on the shells are attributed to polychaete and belong to the ichnogenera Maeandropolydora Voigt, 1965 and Caulostrepsis Clarke, 1908. The aim of this work was to evaluate possible paleoecological relationships between oysters and polychaetes. Xx samples were analized and presence/ausence data were registered on maps of the shells. The maps show different areas established by morphological differences that might influence the epibionts/polychaetes settlement. Most of the borings are parallel to the hinge-ventral margin axis of the valves. ??% of the boring apertures are found on the ventral margin of the left valves. The boring location suggests that the life position of the oyster was with the left valve resting on the mud. The position and number of boring polychaetes close to the ventral margin of the valves sugest that the worms probably took advantage of the currents generated by the oyster. Despite the borings contribute to a weakening of the valve and reduce shell strength, this case seems not to be lethal for the oysters because there is no evidence of borings reaching the interior of the valve from the external surface. In conclusion, it is suggested that there was a probably commensalism symbiotic relationship between Pycnodonte (phygraea) vesicularis and the polychaetes.