A new pleurocystitid rhombiferan echinoderm from the Middle Ordovician Galena Group of northern Iowa and southern Minnesota

Pleurocystites strimplei new species, from the Galena Group of Iowa and Min nesota, is closely related to P. squamosus Billings from the Appalachians a nd Michigan. Numerous specimens provide information about growth, living ha bits, functional morphology, and respiration. The development of P. strimpl ei n. sp. is largely isometric with several exceptions. New dichopores form throughout ontogeny. Length of the dichopores and the area available for r espiration are characterized by strong positive allometry relative to the v olume of the animal. The length of the distal stem increases with respect t o the size of the theca. During life the aboral side faced up. Some animals were largely covered by a thin layer of sediment whereas others lay direct ly on the seafloor The orientation of the brachioles is most consistent wit h deposit feeding. Quantitative models of respiration suggest that the pect inirhombs accounted for over half of the needs of youngsters but this contr ibution falls to about 38 percent in adults. Respiration by the surface are a of the theca and the water vascular system provide small amounts of oxyge n, especially for animals living on the surface of the seafloor. Cloacal pu mping or a similar type of respiratory device probably furnished the remain der of the oxygen required by P. strimplei n. sp. The morphometric data in conjunction with parameters taken from Recent oceans and echinoderms produc e a plausible respiration budget, which is affected by size, age, allometry , and living orientation.