TAPHONOMY OF ‘CYSTOIDS’ (ECHINODERMATA: DIPLOPORITA) FROM THE NAPOLEON QUARRY OF SOUTHEASTERN INDIANA: THE MIDDLE SILURIAN MASSIE FORMATION AS AN ATYPICAL KONSERVAT-LAGERSTÄTTE

Diploporite “cystoids”, like most pelmatozoan echinoderms, are characterized by complex multi-element skeletons prone to rapid disarticulation under normal paleoenvironmental conditions. Consequently, preservation of abundant articulated diploporite thecae is rare and generally restricted to environments where rapid burial events could catastrophically entomb individuals. One such environment is represented by the lower portion of the Wenlock-age Massie Formation at the New Point Stone quarry near Napoleon, southeastern Indiana, which we recognize as a Konservat-Lagerstätte. Taphonomic evidence does not support live burial, however. Diploporites are dominantly preserved as intact thecae with no portions of brachiolar appendages, no oral or anal cover plates, and damage (plate shifting and plate jumbling) to one side of thecae. Post-mortem encrustation by bryozoans and crinoid holdfasts is common, but epibionts are present on the well-preserved sides of thecae. Sedimentary geopetal indicators within thecae show that the well-preserved side commonly faced upward following burial. This taphonomic state is paradoxical, as rapid burial of live diploporites would have resulted in preservation of complete individuals with no encrusters and only compaction-induced damage; rapid burial of diploporites following a short interval of exposure would have resulted in thecae preserved similarly to observed patterns, but with encrusters and geopetal infills indicating that the well-preserved side faced down; and burial after extended exposure would have resulted in thorough disarticulation of thecae. Rather, a variable and, in some cases, complex taphonomic history is suggested for diploporites from the Napoleon quarry, with at least some individuals having experienced one or more brief episodes of exposure prior to final burial. Early diagenetic cementation of initial theca-filling sediment is the most likely mechanism for keeping thecae intact during subsequent exposure.