DIZYGOCRINUS - MISSISSIPPIAN CAMERATE CRINOID (ECHINODERMATA) FROM THE MIDCONTINENTAL UNITED-STATES

The Mississippian crinoid genus Dizygocrinus is revised, and the phylogeny is inferred. Eleven species are recognized, including D. biturbinatus (Hall), D. indianaensis (Lyon and Casseday), D. caroli (Hall), D. whitei (Wachsmuth and Springer), D. originarius (Wachsmuth and Springer), D. montgomeryensis (Worthen), D. gorbyi (Miller), D. venustus (Miller), D. peculiaris (Miller and Gurley), D. mutabilis (Wachsmuth and Springer), and D. cantonensis (Wachsmuth and Springer). Nomenclatoral revisions include recognition of D. gorbyi and D. peculiaris as valid species and designation of the following as junior synonyms: D. calvus, D. crawfordsvillensis, D. intermedius, D. mediocris, and D. somersetensis. Dizygocrinus is now recognized from both the late Osagean and Meramecian on both the eastern and western margins of the Illinois Basin. Dizygocrinus is first recognized virtually simultaneously on both the eastern and western margins of the Illinois Basin. Migration proceeded in both directions, but was most common later in the history of the genus. The principal radiation on the western basin margin was among species with generally smooth aboral cup plates, whereas on the eastern margin sculptured species account for the principal species radiation.