Taphonomy of ‘cystoids’ (Echinodermata: Diploporita) from the Napoleon quarry of southeastern Indiana, USA: The Lower Silurian Massie Formation as an atypical Lagerstätte

Abstract

Diploporites, like most pelmatozoan echinoderms, are characterized by complex multi-element skeletons prone to complete disarticulation if left exposed for extended periods of time. 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. Yet, taphonomic evidence does not support live burial. Diploporites (Holocystites spp.) are dominantly preserved as intact thecae without any portions of brachioles and with damage (plate shifting and plate loss/plate jumbling) on one side. Post-mortem encrustation is common, but epibionts are typically present on the well-preserved sides of thecae. Geopetal sediment-fill of thecae shows that the well-preserved side commonly faced upward following burial. This taphonomic state is paradoxical, as 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 preservation similar to observed patterns, but with encrusters and geopetal infills indicating that the poorly preserved side faced up; 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 exhumation prior to final burial. Early diagenetic cementation of initial theca-filling sediment is the most likely mechanism for keeping thecae intact during subsequent exposure.