The Holocystites fauna of central North America includes most known Silurian Diploporita (Echinodermata: Blastozoa). This distinctive diploporite association, widespread in the Wenlockian of Wisconsin, Illinois, Indiana, Kentucky, Ohio, and Tennessee, consists of eight genera in the Aristocystitidae, Sphaeronitidae, and Holocystitidae. Species of Holocystites Hall, 1864, and Triamara Tillman, 1967, are particularly characteristic. The fauna is best known from the Osgood Member, Salamonie Dolomite (late Llandoverian-early Wenlockian) of southeastern Indiana. Expanded quarrying operations near Napoleon, Ripley County, Indiana, add materially to knowledge of the Holocystites fauna. Thousands of specimens were recovered, including some in life position. Information from this and other localities allows formulation of a paleoecological model for the Holocystites fauna, which is tested against previously known distributional information. Most Silurian diploporites were low-level feeders with relatively inefficient subvective systems as compared with co-occurring camerate crinoids. In the Osgood, they required firm attachment sites in comparatively quiet, offshore, dominantly soft-bottomed environments with a moderate rate of continuous terrigenous sedimentation, conditions limited in southeastern Indiana to the Ripley Island positive area. Two major adaptive strategies are recognized: one relatively eurytopic group comprising species with free adults with elongate thecae, narrow bases, and aboral, stem-like processes, and a more stenotopic group of globular, large-based, completely sessile (attached) species. New taxa include a species of Holocystites Hall, 1864 (H. clavus n. sp.), a new genus (Paulicystis n. gen.) related to Trematocystis Jaekel, 1899, but with uniquely large ambulacral facets, and a new Pentacystis-like genus (Osgoodicystis n. gen.). The fauna also has an advanced sphaeronitid (Finitiporus n. gen.), the only Silurian sphaeronitid yet known. Both the Sphaeronitidae and Holocystitidae are revised. Subfamilies are established in both (Sphaeronitidae: Sphaeronitinae and Herpetocystinae; Holocystitidae: Holocystitinae, Pentacystinae, and Trematocystinae), based largely on peristome morphology. Holocystites is divided into three new subgenera (Holocystites, Megacystites n. sgen., and Sepulticystis n. sgen.) on the basis of pore morphology. Evolutionary trends are noted in the Holocystitidae toward reduction in number of thecal and peristomial plates, enlargement of the subvective system, elevation of the theca. Humatipore morphology becomes more specialized and efficient, but average size decreased. Detailed specific and generic phylogenies are constructed, using both traditional and quantitative phenetic methods. All produced similar results. Osgood diploporite biostratigraphy is revised and a zonal scheme presented. Osgood diploporites are strongly endemic.
