Constructional Morphology of Pelagic Crinoids

While all modern crinoids are benthic filter feeders, some fossil species found in low-oxygen Konservat-Lagersta¨tten probably were 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 passive townet function. However, the short and heavily cirrated stem of another driftwood-dweller (Pentacrinites) suggests active filter feeding. If the buoyant lobolith of Scyphocrinites acted as a swim bladder, this crinoid, by the same tow-net principle, could have made 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 and ligamentary system of modern forms. Nevertheless, they represent distinctive peaks in the adaptive landscape that can be tested using taphonomic, morphological, and evolutionary evidence.