PALEOECOLOGY OF AN ANCIENT SHELL-ENCRUSTING COMMUNITY: OBSERVATIONS FROM "UPSIDE-DOWN" ENCRUSTERS IN EXTERNAL AND INTERNAL MOLDS (UPPER JURASSIC OF SOUTHERN ENGLAND)

Aragonitic shells are usually dissolved away in Mesozoic and older rocks, but often their calcitic encrusters survive dissolution and are preserved "upside-down" within the internal and external molds. These encrusters’ original attachment surfaces are exposed on the molds, allowing us to see growth patterns and living interactions not normally visible from the more common top view. Sometimes these attachment surfaces bioimmured soft-bodied encrusters, revealing a fauna which would have been invisible otherwise. The Portland Stone on the Isle of Portland (Dorset, southern England, Upper Jurassic) contains a diverse fauna of aragonitic and bimineralic bivalves (Laevitrigonia and Isognomon) and ammonites (Titanites). The molds of these mollusks often contain numerous encrusting cyclostome bryozoans (Hyporosopora), oysters (Liostrea), serpulids, foraminiferans (nubeculinellids), rare cheilostome bryozoans, and undescribed stoloniferous bioimmurations. These encrusters (skeletobionts of Taylor & Wilson, 2002) were apparently cryptic, with some inhabiting the interiors of nearly-closed articulated shells. Ecological succession can be deduced by recording the stratigraphic order of encrustation. Serpulids were often the pioneers on these shells, followed by bryozoans and then oysters. Living interactions between some encrusters are evident from changes in growth patterns and directions. The attachment surfaces of some upside-down encrusters are themselves encrusted, meaning that the aragonitic shell originally hosting them dissolved on the seafloor during the lifespan of the community. Similar upside-down encrusters can be found in other faunas containing abundant aragonitic molds, such as the Cincinnatian of North America. These should also reveal details of hard substrate paleoecology not detectable on upper surfaces.