NAUTILOID CEPHALOPODS AS SUBSTRATA FOR ENCRUSTATION BY STALKED ECHINODERMS IN THE MIDDLE SILURIAN (WENLOCK) MASSIE FORMATION OF SOUTHEASTERN INDIANA

The middle Silurian (Wenlock: Sheinwoodian) Massie Formation as exposed at the New Point Stone quarry near Napoleon, Ripley County, southeastern Indiana, is a richly fossiliferous unit that is highly paleoecologically and sedimentologically significant owing to pervasive regional dolomitization. Nautiloid cephalopods are common but comprise a low diversity fauna dominated by Dawsonoceras annulatum, with far subordinate contributions from indeterminate orthoconic forms and Michelinoceras, both of which are preserved primarily as internal molds. Approximately 8% of cephalopod specimens are encrusted by one or more pelmatozoan echinoderm attachment structures, with patterns of encrusted substrata corresponding to the overall taxonomic and taphonomic (moldic vs. non-moldic) composition of the assemblage. Echinoderm attachment structures consist of, in decreasing relative abundance, (1) minute, solid, discoidal structures, possibly belonging to juvenile crinoids or coronoids; (2) dendritic radices, attributable to the camerate crinoid Eucalyptocrinites and the hemicosmitid rhombiferan Caryocrinites; (3) enigmatic multilobate structures most likely belonging to crinoids; (4) large, multiplated, pore-bearing discoidal structures representing aboral thecal attachments of holocystitid diploporitans; and (5) coiled dististeles attributable to the crinoid Dimerocrinites. Patterns of echinoderm encrustation of nautiloids provide two significant insights into the sedimentological and paleoecological history of the Massie Formation. First, the morphology of encrusting structures shows that, at least in some cases, pelmatozoans were capable of employing a hardground-style attachment strategy when encountering large bioclasts in softground settings. Second, encrustation of internal molds provides explicit evidence for exhumation of temporarily buried cephalopods. Biostratinomic reorientation is further evidenced by encrustation of specimens with one damaged hemisphere, presumably reflecting surficial degradation of the upward-facing side of partially buried cephalopods; in these instances, encrustation of the more well-preserved side requires overturning.