Systematics and phylogeny of the late Osagean and Meramecian crinoids Platycrinites and Eucladocrinus from the Mississippian stratotype region

Abstract

Late Osagean to Meramecian Platycrinites and Eucladocrinus from Illinois, Iowa, and Missouri are redescribed and redefined from study of type material. Three of the oldest species were incorrectly synonymized with P. sarae, and this correction leads to a realignment of species systematics within Platycrinites. Accordingly, six species of Platycrinites and one species of Eucladocrinus are considered valid in the Keokuk Limestone through St. Louis Limestone of the stratotype area. Valid species include Platycrinites saffordi (Hall), P. sarae (Hall), P. georgii (Hall), P. pumilus (Hall), P. brevinodus (Hall), P. niotensis (Meek and Worthen), and Eucladocrinus millebrachiatus Wahsmuth and Springer. Nomenclatoral changes include the following: 1) P. sarae is redescribed and all junior synonyms are removed; 2) P. georgii is a valid senior synonym of P. bonoensis (White), P. aeternalis (Miller), P. boonvillensis (Miller), and P. cauducus; 3) P. pentagonus (Miller) is a junior synonym of P. niotensis; 4) P. pumilus is a valid senior synonym of P. prattenanus (Meek and Worthen), and P. monroensis (Worthen); 5) P. bloomfieldensis (Miller) is designated as a nomen dubium; and 6) Eucladocrinus millebrachiatus immaturus is a junior synonym of E. millebrachiatus.The evoluton of new species in Platycrinites was by a combination of cladogenesis and possibly anagenesis. Platycrinites sarae evolved from P. saffordi possibly through anagenesis, P. pumilus evolved from P. niotensis through cladogenesis, and P. hemisphaericus evolved through cladogenesis from P. brevinodus, although P. hemisphaericus does not occur in the geographic area of this study.