The relations between allometry, phylogeny, and functional morphology in some calceocrinid crinoids

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Abstract
Calceocrinids are unique crinoids with the crown movably hinged to a recumbent stem. The crown was raised for feeding by closing the hinge and bending the proximal stem. While resting, the crown lay parallel to the stem with the hinge opened. Allometric equations for four species reflect a combination of ontogeny, adult body size, phylogeny, habitat, and functional morphology. The hinge of most adult calceocrinids is extended into ear-like projections which made the crown more stable on the seafloor. For the hinge moment versus the effective weight of the crown, positive allometry characterized taxa that lived in agitated environments, whereas isometry was adequate for quiet-water species. The adult body size provides a secondary effect on the data for crinoids from the same type of environment. Here, the initial intercepts are transposed so that larger animals functioned like smaller ones. The equations are independent of phylogenetic position. All species exhibit positive allometry of the length and number of plates in the arms. The food-gathering capacity of a crinoid is estimated by the number of food-catching tube-feet multiplied by the width of the food grooves, whereas the soft parts that must be fed are proportional to the crown volume. The ratio of food-gathering capacity to crown volume is either fixed or decreases slightly in larger crinoids. Statistical tests reveal that all species follow a single developmental pattern for these two parameters. However, some of the evolutionary changes in the arms permitted calceocrinids to retain an adequate food-gathering capacity into larger adult body sizes.