Abstract

The lichenocrinid holdfast of Cincinnaticrinus varibrachialusWarn and Strimple, 1977 was attached to a variety of hard objects, i.e., crinoid stems, twig-shaped and flat bryozoans, brachiopods, bivalves, cephalopod shells, trilobites, and phosphate nodules, throughout life. This represents an adaptation to a soft substrate and most individuals are found in carbonate or carbonate-clay muds that were deposited in moderately deep and quiet water. Evidence from living and fossil crinoids suggests that the cincinnaticrinid column either required little nourishment from the crown or it may even have been metabolically self sustaining. The food-gathering system mainly needs to supply the energy needs of the crown with little or no contribution to the stem. Very likely, this applies to many crinoids. Consequently, it is appropriate to analyze the food gathering system with respect to the crown volume as a size parameter.

The aboral cup grows slowly compared to the arms. Brachs and ramulars show little shape change. Length of the column is augmented by formation of new columnals and size increase of old ones at all growth stages. Virtually all new columnals form by intercalation between older plates rather than being initiated immediately below the aboral cup. Most size-related variables in the arms are characterized by positive allometry relative to crown volume. New arm branches develop continuously. Likewise, new brachs and ramulars appear at the distal tips of all arms. This plate supply rate along with deposition of calcite on old arm plates produces an exponentially increasing growth rate for total length of the arms. Food-gathering capacity equals the number of food-catching tubefeet times the average food groove width. Due to the positive allometry of the size of the arms, the ratio of food-gathering capacity:crown volume is constant regardless of size so the capacity of the food-gathering system keeps pace with the tissue that must be fed. However, the average food groove width only changes slightly so that youngsters and adults ate food particles of about the same size.

Variation in number of brachs in the arms of C. varibrachialus is comparable to that of many Ordovician cladids. Specimens of C. varibrachialus from the Kope Formation of the Cincinnati, Ohio, area possess more numerous primibrachs than animals from the Walcott-Rust Quarry, although the two populations overlap greatly. Intercalated brachs are known in the midcontinent crinoids but not in the New York specimens. The basic arm structure, cup and crown shapes, ornamentation, and the morphology of the columnals and the holdfast of all crinoids are identical and the midcontinent and New York suites of crinoids are considered conspecific.

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