Cupulocrinid crinoids from the Middle Ordovician (Galena Group, Dunleith Formation) of northern Iowa and southern Minnesota

Five species of camerate crinoids from the Middle Ordovician Dunleith Formation (Galena Group) of northern Iowa and southern Minnesota are described: Cleiocrinus regius Billings, Cotylacrinna sandra n. gen. and n. sp., Euptychocrinus skopaios n. gen. and n. sp., Abludoglyptocrinus charltoni (Kolata), and Eopatelliocrinus ornatus (Billings) n. comb. Archaeocrinus desideratus Billings is assigned to Cotylacrinna, and Glyptocrinus fimbriatus Shumard and Ptychocrinus longibrachialis Brower are placed in Euptychocrinus. The geographical affinities of the Iowa and Minnesota crinoids lie with adjacent localities in the northern midcontinent and the Appalachian province of Canada and New York. Preservation of various specimens implies that Euptychocrinus skopaios, Abludoglyptocrinus charltoni, and Eopatelliocrinus ornatus utilized parabolic arm fans. Cotylacrinna sandra is a specialized rhodocrinitid which is perhaps the largest completely known Ordovician crinoid with a stem length of over 91 cm and a total volume of about 45,740 mm3. The root morphology indicates an upright column and this animal towered above the associated echinoderms, which ranged from the substrate level to a maximum of 25 cm above the seafloor. Euptychocrinus skopaios is marked by dwarfed morphology compared to the closely allied E. fimbriatus, and it exhibits accelerated development of fixed brachs, number of brachials in and length of the arms, and closely spaced pinnules. The dwarfism is interpreted as a specialization for small size and adults of E. skopaios were only located about five or six cm above the substrate. New brachials and pinnules form at the distal arm tips of E. skopaios throughout ontogeny. Consequently, the length of and the number of plates in the food-gathering system are positively allometric relative to the crown volume. Food gathering capacity equals the number of food-catching tube feet times width of the food grooves and it is also augmented more rapidly than expected for an isometric crinoid. Although distantly related to euptychocrinids, most other Dunleith camerates, namely Abludoglyptocrinus charltoni, Eopatelliocrinus ornatus, and Cotylacrinna sandra, follow the same developmental trends of the food-gathering system observed in Euptychocrinus skopaios. Comparison of the pinnulate camerates and three species of ramulate cladids and a disparid from the Dunleith reveals some striking contrasts. At equivalent crown volumes, the camerates are characterized by more numerous arm branches in the form of pinnules, more narrow food grooves, more closely spaced tube feet, and longer food-gathering systems with more plates and greater capacity. The Dunleith camerates were adapted for catching smaller food particles using more numerous and more closely spaced tube feet located on more extensively branched filtration nets than the associated cladids and disparid. The differences can be attributed to taxonomy and presumably phylogeny, that is camerates versus cladids and disparids, and/or morphology in the presence of pinnules versus ramules.