Bioerosion traces by endolithic foraminiferans on Recent and Miocene oysters from Patagonia (Argentina)

ROMERO, M.V.; FERRERO, L.; BREZINA S.S.; BREMEC, C.S.; CASADÍO, S.

General Roca

Congreso; 11º CONGRESO DE LA ASOCIACIÓN PALEONTOLÓGICA ARGENTINA; 2016

This study reports the identity of foraminiferans within rosetted borings assigned to Semidendrina pulchra (Bromley, Wisshak, Glaub, Botquelenon, 2007) on the Recent oyster Ostrea puelchana d´Orbigny, 1842, from San Matías Gulf (Patagonia, Argentina). Foraminiferan tests were removed from their bioerosion traces and, SEM (Scanning Electron Microscope) and SEM-EDX (energy dispersive X-ray) analyses were performed in order to assess their morphological and compositional features. The same rosetted borings were recognized on the Miocene species Ostrea alvarezii d´Orbigny, 1842, from Puerto Madryn Formation (Patagonia, Argentina, MPEF-PI 5001-5376). S. pulchra on Recent and Miocene oysters were studied with SEM in low vacuum mode. The traces were clustered in large numbers on valves of both oysters. They consisted of a fan-shaped plexus of galleries, which were connected to the surface by perforations and, for the other side, with a main chamber where an individual specimen was nested. These rosetted borings are dwelling or nesting traces produced by a weakly agglutinated foraminiferan assigned tentatively to genus Trochammina Parker and Jones, 1859. Miocene and Recent oysters lived in comparable environmental conditions, mainly lying on the right valve and living in tidal channels with shelly-sandy bottoms. Both oysters present similar morphological areas within the valves and S. pulchra colonized preferentially the same areas clear of sediment (i.e, apex and center) and they showed also similar sclerobiontic assemblages associated on valves. Considering this, it could be proposed that the traces assigned to S. pulchra recorded on Miocene O. alvarezii valves may have a similar foraminiferan producer.