INTERACTIONS BETWEEN CORALS AND STALKED ECHINODERMS IN THE MIDDLE SILURIAN (WENLOCK: SHEINWOODIAN) MASSIE FORMATION OF SOUTHERN INDIANA

Stalked echinoderms are abundant within middle Silurian (Wenlock) strata exposed at the New Point Stone quarry east of Napoleon, Ripley County, southeastern Indiana. At this locality, the lower decimeter of the mudstone lithofacies of the Sheinwoodian-age Massie Formation is recognized as a konservat-Lagerstätte owing to the articulated state of multi-element skeletons, including a diverse assemblage of crinoids and diploporitan “cystoids.” Among the numerous articulated thecae and intact column segments are multiple specimens that show unambiguous evidence of syn vivo interactions between stalked echinoderms and corals; such biotic associations have not hitherto been described in detail. Two primary forms of coral-pelmatozoan interactions are present. The first involves encrustation of vertically oriented portions of the column of the monobathrid camerate crinoid Eucalyptocrinites by colonies of favositid tabulate corals (probably Favosites). Coral colonies encrusted the entirety of the lateral circumference of the crinoid column segment. The corals presumably benefitted via elevation above the muddy seafloor. The crinoid hosts displays no pathologic response to encrustation, suggesting the absence of an adverse effect; this most likely reflects the unusually strong and rigid Eucalyptocrinites column, which could support heavy encrusters. The second interaction involves encrustation of columns of the hemicosmitid rhombiferan Caryocrinites by thin, laminar rugosan colonies. The “cystoid” hosts are strongly pathologically deformed, displaying malformations and severe swellings of secondary stereomic calcite around the corals. These corals also likely benefitted from elevation above the surrounding seafloor, as in the first described interaction, but development of galls by Caryocrinites indicates a negative reaction by the echinoderm host. Other coral-encrusted echinoderm specimens—including all involving solitary corals—reflect post mortem encrustation of echinoderm bioclasts.