ABSTRACT
While all modern crinoids are benthic filter feeders, some fossil species found in low-oxygen KonservatLagerstaetten have been probably pelagic. Pseudoplanktonic forms were mostly attached to driftwood (Seirocrinus, Traumatocrinus, Melocrinus); they had long, rope-like stems and enlarged, permanently splayed filter fans, as required for a tow-net function. However, the short and heavily cirrated stem of another driftwooddweller (Pentacrinites) suggests active filter feeding. If the buoyant lobolith of Scyphocrinites acted as a swim bladder, this crinoid could, by the same tow-net principle, make use of velocity gradients near a boundary layer. Paradigms are different for stemless forms, whether they floated passively over the bottom (Uintacrinus) or filtrated actively in the water column (Roveacrinids and Saccocoma). Some of the proposed models could not function with the muscular system of modern forms. Nevertheless they represent distinctive peaks in the adaptive landscape that can be tested using taphonomic, morphological, and evolutionary evidence.
