Paleoecology and taphonomy of two new edrioasteroids from a Mississippian hardground in Kentucky

Abstract

A recently discovered series of hardground surfaces in the lower Salem Limestone (Meramecian, Mississippian) and upper Muldraugh Member of the Borden Formation (upper Osagean to lower Meramecian, Mississippian), in Hardin County, Kentucky is colonized by a hard substrate community that includes three species of pyrgate edrioasteroids. Torquerisediscus kypsi n. gen. and n. sp. differs from other edrioasteroids by bearing a cyclic pattern of twelve ambulacral cover plates and meandering distal ambulacra. Ulrichidiscus spinosus n. sp. differs from other species of Ulrichidiscus by the presence of large spines on the ambulacral cover plates, anal pyramid plates, and hydropore orals. Two small specimens are assigned tentatively to Ulrichidiscus aff. pulaskiensis.Edrioasteroid populations occurring on several surfaces show marked differences in size frequency distribution. Three of the surfaces only have large individuals of Torquerisediscus kypsi whereas a fourth surface is colonized by only juvenile specimens of T. kypsi as well as both species of Ulrichidiscus suggesting subtle differences in environment and duration of surface colonization prior to burial. Furthermore, taphonomic pathways differ among these four surfaces with some containing only disrupted individuals and others containing only fully articulated individuals suggesting either differences in burial depth, or some differences in decomposition of edrioasteroid carcasses prior to final burial.