STUDYING THE NATURALLY-EXPOSED ATTACHMENT SURFACES OF FOSSIL ENCRUSTERS (SKELETOZOANS): A SIMPLE ACETATE PEEL TECHNIQUE REVEALS GROWTH HISTORIES AND SURFACE PALEOECOLOGY

Skeletal encrusters on shells (skeletozoans of Taylor & Wilson, 2002) sometimes are preserved "upside-down" with their attachment surfaces exposed if the original shell substrate has been removed. This is especially common if the skeletozoan is calcitic and it encrusted an aragonitic shell layer which dissolved. The results are rare and useful views of the undersides of skeletozoans such as bryozoans, serpulids, oysters, brachiopods, cornulitids, barnacles, and foraminiferans. Since these attachment surfaces are almost always naturally etched, acetate peels can usually be made from them without polishing and immersing in acid. These peels reveal, often with exquisite detail, the otherwise-hidden growth histories of these skeletozoans across the shell substrates. Bryozoans, for example, commonly show astogenetic patterns from the ancestrulae to the last growth margin, patterns which are usually obscured on the upper surfaces by later encrustation. The initial orders of skeletozoan succession are also shown in this reversed view, including small organisms which would have been completely invisible beneath larger encrusters, and some soft-bodied forms which left bioimmurations in the skeletons of those which covered them. Competitive interactions between contemporaneous skeletozoans can also be observed, such as thickened skeletons at margins and abrupt changes in growth directions or patterns. The value of this "upside-down" perspective on skeletozoan communities is demonstrated with examples from Upper Ordovician (Cincinnatian) and Upper Jurassic (Portlandian) shallow marine faunas.