Abundance and distribution of Lithophaga (Mytilidae) in extant and fossil oysters: taphonomic and paleobiological implications

In this study we analyze the abundance and distribution patterns of Lithophaga patagonica in valves of Ostrea puelchana and compare them to those of Lithophaga sp. observed on the fossil species "Ostrea" patagonica and "Ostrea" alvarezii from the late Miocene Puerto Madryn Formation. No specimen of the fossil oysters showed borings of Lithophaga sp. on the interior surface of the valves. This suggests that they were produced while the oysters were still living and, at the same time, that the oyster beds were buried rapidly after death. In Ostrea puelchana the boring abundance was significantly higher for the left valve and, within it, the areas more heavily bored were the umbones and the platform. The same results were obtained for valves of "Ostrea" alvarezii, suggesting that this oyster showed life habits similar to the living one. On the other hand, in "Ostrea" patagonica the abundance of Lithophaga borings was the same on both valves. This agrees well with its life habit, in which the shells mainly are oriented almost vertically. The left valve of "Ostrea" patagonica showed no preferential location for the borings. In the right valves of "Ostrea" patagonica the posterior and anterior margins show perforation values that are higher than expected. Results suggest that the life position of oysters is one of the factors influencing the abundance and distribution of Lithophaga borings. This information is useful to infer the life position of fossil oysters and to reconstruct their taphonomic history.