EARLY CEMENTATION OF THE BRACHIOPOD BOUCHARDIA ROSEA ON A MODERN TROPICAL SHELF

Shallow, tropical shelf settings of the Southeast Brazilian Bight are home to large populations of the articulate ( = rhynchonelliform ) brachiopod Bouchardia rosea, and provide a unique modern analogue for ancient brachiopod-dominated fossil assemblages. Specimens collected from the vicinity of Ilha Grande include a small population (43 individuals) of subfossil shells with a distinctive mode of preservation, showing early diagenetic cementation. The formation of sparry cement within the shell interior preserves both valves in their original articulated position, severely restricting opportunities for post-mortem colonization by hard-substrate encrusting organisms. Organic tissues of the pedicle are sometimes preserved within sparry cement filling the pedicle opening, indicating rapid onset of crystallization. The limited taphonomic alteration of these shells, particularly with respect to encrustation and bioerosion, and the preservation of organic tissues indicate the rapid onset of cementation, most likely through rapid burial below the taphonomically active zone. Detailed taphonomic analysis of these specimens may provide greater understanding of preservation processes in the fossil record, particularly shell geodization and the formation of geopetal structures.