Rotational stability in stalked crinoids and the function of wing plates in Pterotocrinus depressus

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Abstract
An unusual feature of the Mississippian camerate crinoid Pterotocrinus depressus is its tegmen with five wing-like appendages (wing plates). It has been suggested that the wing plates served as anti-predatory devices, hydrodynamic baffles, or outriggers for stabilization and support on the substrate. We propose two alternative hypothesis. First, the wing plates may have served as splitter plates, which would have acted to reduce drag on the organism. Second, wing plates served as stabilizing fins or rudders, allowing the passive maintenance of an efficient feeding posture in moving water. Experimental analyses of the forces acting on model crinoids, with and without the wing plates, support the hypothesis of the rudder function. A model developed for maintenance of stability in moving water should be applicable to all suspension feeders.