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

Abstract

Pleurocystites strimpleinew species, from the Galena Group of Iowa and Minnesota, is closely related toP. squamosusBillings from the Appalachians and Michigan. Numerous specimens provide information about growth, living habits, functional morphology, and respiration. The development ofP. strimplein. sp. is largely isometric with several exceptions. New dichopores form throughout ontogeny. Length of the dichopores and the area available for respiration are characterized by strong positive allometry relative to the volume of the animal. The length of the distal stem increases with respect to 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 directly on the seafloor. The orientation of the brachioles is most consistent with deposit feeding. Quantitative models of respiration suggest that the pectinirhombs accounted for over half of the needs of youngsters but this contribution falls to about 38 percent in adults. Respiration by the surface area of the theca and the water vascular system provide small amounts of oxygen, especially for animals living on the surface of the seafloor. Cloacal pumping or a similar type of respiratory device probably furnished the remainder of the oxygen required byP. strimplein. sp. The morphometric data in conjunction with parameters taken from Recent oceans and echinoderms produce a plausible respiration budget, which is affected by size, age, allometry, and living orientation.