PALEOECOLOGY OF AN UPPER JURASSIC CRYPTIC COMMUNITY INHABITING EMPTY MOLLUSK SHELLS (PORTLAND LIMESTONE OF SOUTHERN ENGLAND)

The preservation of numerous exquisite molluscan internal molds in the Upper Jurassic Portland Limestone on the Isle of Portland in southern Dorset, England, has allowed us to reconstruct a filter-feeding encrusting skeletozoan community which lived inside the shells prior to final burial. The shells include conchs of large ammonites (Titanites), aragonitic and bimineralic clams (Laevitrigonia and Isognomon), and small oyster-like bivalves (Plicatula). The encrusters fixed themselves onto the inner surfaces of the empty shells; their attachment surfaces were exposed when the internal mold separated from the shell. In most cases this was done naturally by early dissolution of the molluscan aragonite and calcite cementation of the internal mold. In the calcitic oysters the molds can be extracted as steinkerns from the surviving articulated shells. The skeletozoans include a common cyclostome bryozoan (Hyporosopora), serpulids, foraminiferans (nubeculinellids), rare cheilostome bryozoans, and undescribed stoloniferous bioimmurations preserved most commonly underneath small cryptic Liostrea. Because the encrusters are preserved "upside-down" in these molds, we can deduce their growth habits along the shell surface. For example, the cyclostome bryozoans developed frondose growth and peripheral subcolonies in response to microenvironmental stresses. Serpulids may have employed a straight-line "running" growth form to escape envelopment by bryozoans and oysters. A clear ecological succession is not evident, probably because larval emergence was staggered over seasons. However, a typical pattern of overgrowth shows serpulids as pioneers encrusted by bryozoans which were in turn covered by oysters. There is also a gradient of crypticness, with Foraminifera apparently favoring the deepest recesses of Laevitrigonia by fixing on the inner hinges. Besides these skeletozoans, ophiuroids also appear to have inhabited these empty shells.