The oyster Crassostrea? hatcheri (Ortmann) is one of the most common fossils in Oligocene–Miocene marine rocks in Patagonia, southern Argentina. This oyster is distinguished by its large size and the great thickness of its valves. It built framework reefs in shallow-shelf environments, and its valves form large biogenic and sedimentologic concentrations exposed in many areas of Patagonia. This paper focuses on the community of boring and encrusting organisms living on the valves of C.? hatcheri, in both the biogenic and sedimentologic concentrations. This community includes fungi, algae, sponges, arthropods, bryozoans, phoronids, polychaetes, brachiopods, and mollusks. The recorded diversity is comparable to that observed in Recent oyster reefs, suggesting that C.? hatcheri was a physical ecosystem engineer in shallow-shelf environments during the late Oligocene–early Miocene. Primary factors in their role as facilitating organisms were, besides the size of the valves, the age of adult specimens (average = 25 years), high population densities, wide geographical range (more than 10° latitude), and long temporal range (more than 5 My), in addition to the framework of the reefs and the sedimentologic concentrations of the valves.

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