PALEOBIOLOGY OF AN ENIGMATIC CRINOID (ECHINODERMATA) FROM THE PENNSYLVANIAN VANPORT LIMESTONE, NORTHEASTERN OHIO

The Middle Pennsylvanian Vanport Limestone represents a lithologically and faunally distinctive transgressive unit (“marine zone”) in the Allegheny Group of the Appalachian Basin.  In general, this unit is relatively thin and geographically discontinuous, but the basal portion of the Vanport is exceptionally well exposed in a small quarry in northern Holmes County, northeastern Ohio. This interval has historically been recognized as a source of crinoid material, preserved exclusively as isolated columnals, pluricolumnals, and partial columns; however, the exposure studied here contains numerous articulated columns (numerous specimens exceeding 30 cm in length), permitting the first detailed paleobiological and paleoecological analyses of enclosed crinoids. The lower Vanport contains a monospecific assemblage composed of a taxon for which the crown in unknown; hence, systematic placement is currently impossible, although a camerate or cladid affinity is most likely. Columns are xenomorphic, large-diameter, unusually long for Paleozoic taxa (maximum length greater than 75 cm), and inflexible. The proxistele is unknown; the mesistele is weakly heteromorphic with sparse radices and a noditaxis pattern suggesting distribution of ligamentary connective tissues comparable to those of modern stalked crinoids; the dististele is characterized by numerous, radially or bilaterally distributed, dichotomously branching radices, indicating modification into a dendritic attachment structure. Development of a radicular attachment structure allowed colonization of the soft, fine-grained, micritic sediments of the basal Vanport Limestone. Increased vertical column length may be related to intermittent benthic oxygen fluctuations. Collectively, these data indicate that the crinoid fauna of the lower Vanport Limestone consists of a single soft substrate-adapted species with a long, inflexible column, possibly prone to crown and/or holdfast autotomy.