Four species of isorophinid edrioasteroids, Ulrichidiscus forbesi n. sp., Neoisorophusella whitesidei n. sp., Postibulla westergaardi n. sp., and Parapostibulla graysoni n. sp., are described from unnamed unit 1 of the Golf Course Formation, Dornick Hills Group, Lower Pennsylvanian (Morrowan), at Forbes Ranch, north of Marietta, southern Oklahoma. This locality is the first report of edrioasteroids attached to extraformational clasts, and they exhibit extremely high specimen density and the highest taxonomic diversity known from the post-Ordovician. The associated fauna consists of the stemless crinoid Paragassizocrinus tarri, serpulid worm tubes, and the acrothoracic barnacle boring, Rogerella, along with a few brachiopods, rugose corals, and other open marine organisms. Edrioasteroids show no preference for clast type, size, or shape. Intraspecific spatial competition is shown by polygonal specimen outlines in highly crowded conditions, whereas interspecific competition results in specimen overgrowth in a predictable manner.