MORPHOLOGY AND OCCURRENCE OF A SPHAERONITID DIPLOPORITAN (ECHINODERMATA) ATTACHMENT STRUCTURE IN THE MIDDLE SILURIAN OF SOUTHEASTERN INDIANA

A distal attachment structure, attributable to the sphaeronitid diploporitan Finitiporus boardmani Frest and Strimple, 2011, is described herein from a hardground surface in the middle Silurian (Wenlock: Sheinwoodian) Massie Formation at the Napoleon quarry of southeastern Indiana. This surface is known to be densely encrusted by pelmatozoan echinoderms, with distinctive holdfast assemblages located in three sub-environments on the hardground: elevated hardground crests, partially sediment-filled hardground troughs, and fistuliporoid-micrite microbioherms. The specimen described here is not directly attached to any substrate and is present in rubbly material immediately adjacent to a microbioherm, suggesting dislodgement from atop the build-up followed by very local transport. The F. boardmani attachment structure consists of a small number of elongate plates lacking diplopores. The structure tapers slightly distally before expanding into a terminal, circular disk. The proximal region is incomplete and no portion of the theca is articulated to the specimen; nevertheless, the morphology of the isolated holdfast is sufficiently distinct to permit identification. In total, the occurrence of the F. boardmani holdfast raises the overall diversity of stalked echinoderm attachment structures on the Massie Formation hardground from twelve to thirteen; it represents an additional component of the microbioherm-encrusting assemblage and the first discoidal morphology reported from this portion of the Napoleon quarry; and it allows detailed description and interpretation of the frequently-overlooked aboral region of a blastozoan echinoderm.