CUPULOCRINID CRINOIDS FROM THE MIDDLE ORDOVICIAN (GALENA GROUP, DUNLEITH FORMATION) OF NORTHERN IOWA AND SOUTHERN MINNESOTA

Two cupulocrinids, Cupulocrinus crossmani n. sp. and Praecupulocrinus conjugans (Billings) n. gen., are known from the Middle Ordovician (Galena Group, Dunleith Formation) of northern Iowa and southern Minnesota. Various morphologic and ontogenetic features demonstrate that Praecupulocrinus is more primitive than Cupulocrinus. The two species commonly occur together. In addition, both taxa coexisted at similar levels with stem lengths ranging from about 1.5 cm in juveniles to 15 cm in adults. Relatively complete growth sequences illustrate growth and variation and show how two related crinoids subdivided feeding niches. The crown volume provides a satisfactory surrogate variable for the size of the animal. The food-gathering system of the cupulocrinids is mainly augmented by the addition of new plates at the ends of the arms. The number of plates in the arms and the arm length exhibit positive allometry relative to crown volume, largely due to development of new branches at the arm tips. The food-gathering capacity equals the number of food-catching tube-feet multiplied by the average width of the food grooves. Food-gathering capacity is also positively allometric with respect to crown volume and the amount of tissue that must be supplied with food. Consequently, the ratio of food-gathering capacity:crown volume is either constant or declines slightly with increasing size and age. The food groove width increases throughout ontogeny so adult crinoids ate larger food particles than juveniles. Praecupulocrinus conjugans (Billings) n. gen. has more narrow food grooves than Cupulocrinus crossmani n. sp. of comparable size and age, which suggests niche differentiation based on food-particle size. The arm and tube-foot geometry indicates that both cupulocrinids utilized the same type of suspension feeding. The morphology of the anal sac and the lack of "patelloid" processes in the arms indicate that Cupulocrinus sepulchrum Ramsbottom from the Upper Ordovician of Scotland belongs to Dendrocrinus.