A NEW GENUS OF BELEMNOCYSTITID SOLUTE (ECHINODERMATA) FROM THE LEBANON LIMESTONE (UPPER ORDOVICIAN, SANDBIAN) OF CENTRAL TENNESSEE: RECONSTRUCTION USING PHASE-CONTRAST X-RAY MICROTOMOGRAPHY

The Soluta are a clade of blastozoan echinoderms which are characterized by an ovoid, arrow-shaped, or somewhat irregular body covered in tessellated polygonal plates. The enclosed body, or theca, possesses two appendages: a double biserial feeding arm, or brachiole, and a homoiostele, which is typically longer than the theca and is divided into a shorter but more flexible proxistele and a longer, more rigid dististele. Belemnocystitids comprise a group of solutes distinguished by a relatively small number of regularized thecal plates. Belemnocystitidae is restricted to the Upper Ordovician of Laurentia and previously contained only two monospecific genera. These solutes have a dorsoventrally flattened theca with a brachiole on the dorsal surface, situated next to an inferred hydropore. The ventral surface possesses a cycle of marginal plates around several somatic plates.

In this study we describe a new genus of belemnocystitid from the Lebanon Limestone (Upper Ordovician, Sandbian, Mohawkian) of central Tennessee. The specimen was collected from a 5 cm-thick wackestone slab from float which, based on lithology, likely occurred in the lower member. The ventral surface of gen. et sp. nov. is exposed on the surface of the slab, but the homoiostele is absent. This taxon is distinguished by a reduced number of enlarged marginal plates, proportionally smaller ventral somatic plates, and an absent A4 ventral. We used phase-contrast X-ray microtomography to reconstruct the buried dorsal surface and brachiole. Using these new data, we propose a phylogeny of the Belemnocystitidae and discuss implications for paleobiogeography. This group is known from the Sandbian of Oklahoma, Tennessee, and Ontario and the lowermost Katian of Kentucky. The recently-discovered occurrence in the Lebanon Limestone provides new insight into the dispersal of the group and preferences for hardground-dominated environments.