380 The North American Holocystites Fauna

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. 

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. 

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.