Eoparisocrinid crinoids from the Middle Ordovician (Galena Group) of northern Iowa and southern Minnesota

Two species of eoparisocrinid crinoids from the Middle Ordovician Galena Group of northern Iowa and southern Minnesota are described, namely Eoparisocrinus crossmani n. sp. and E. grandei n. sp. The post-larval development of Eoparisocrinus crossmani is examined. Crinoid arms grow by addition of new plates at their distal tips in conjunction with calcite deposition on old plates. New branches appear where axillary plates are initiated. Consequently, the growth rates for number of brachials and length of food-gathering system compared to crown volume are much faster than if the animals were isometric. The number of food particles collected is related to the number of food-catching tube-feet, which can be estimated if the length of the arms and height of the covering plates are known. The size of the largest food item is constrained by the food groove width. Thus, food-gathering capacity is the number of food-catching tube-feet multiplied by food groove width. The food-gathering capacity increases more rapidly than if the animal grew isometrically, and the ratio of food-gathering capacity: crown volume only declines slightly over the known growth range. All Ordovician cladid crinoids examined follow nearly identical ontogenetic trajectories. The ecological niche of a stalked crinoid is related to four basic parameters: stem length, food groove width, tube-foot spacing, and branch density. Stem length limits the highest elevation above the seafloor. The column of E. crossmani becomes longer during ontogeny due to the formation of new columnals and height growth of old ones. Consequently, individuals gradually “move up” until the adult elevation of about 50 mm is reached. The growth rates of stem length relative to crown size are slow in the youngest and mature animals but rapid in juveniles. The food grooves become wider throughout growth so that older crinoids ate larger food particles than younger ones. The food groove width increases less rapidly than if the shape were constant, because distal plates and branches are more narrow and have more slender food grooves than proximal plates. Growth curves for food groove width versus stem length and elevation were generated for E. crossmani and other crinoids that commonly occur in the same beds. Together, elevation and food particle size define the main dimensions of the niche. The various taxa are more or less separated by different food groove widths at most comparable elevations. This pattern minimizes ecological overlap and probably competition between the different species. The tube-foot spacing of E. crossmani is constant regardless of size, which suggests that it employed the same type of feeding mechanism throughout post-larval ontogeny. The arm branches of adults gradually become less densely spaced relative to the area of water filtered than in juveniles.