Echinoderms of the Somerset Shale Member, Salem Limestone (Mississippian), in Indiana and Kentucky

Abstract

The Somerset Shale Member (Mississippian: Valmeyeran) is a discontinuous shale at the base of the Salem Limestone in southern Indiana and central Kentucky. It is the only significant terrigenous unit within a thick sequence of shoaling carbonates deposited on the eastern side of the Illinois Basin over the Borden Delta siltstones. The Somerset Shale contains a diverse echinoderm fauna of blastoids (five species), crinoids (20 species), and echinoids (two species). Most of the species are known only from the Somerset or from carbonate rocks directly above and below the Somerset Shale. There remains a small number of species previously known mostly from terrigenous rocks (the Borden Delta) below the carbonates. This demonstrates that at least some of the faunal differences between the Borden crinoids and those of the overlying carbonates are the result of paleoecologic factors. The Somerset Shale may mark a useful biostratigraphic boundary defined by last occurrences of the genera Barycrinus, Cyathocrinites, and Actinocrinites and the base of the range of Batocrinus. The species Batocrinus somersetensis n. sp., Dizygocrinus calvus n. sp., and Barycrinus punctus n. sp. are described.