USING THE DUAL REFERENCE HOMOLOGY SYSTEM AND OTHER HOMOLOGOUS STRUCTURES TO STUDY MORPHOLOGICALLY DIVERGENT AND PROBLEMATIC CRINOIDS

A dual reference system of primary crinoid cup plate homologies (DRHS) has recently been published by the authors along with descriptions of the oldest known crinoids. The new homology system is offered as an alternative to single reference plate homology systems including the traditional system of Carpenter (1879). Rationale for the new system is based largely on two of these oldest crinoids recognized as protocrinoids. The protocrinoid cup is characterized by: 1) partial organization including a cup-base circlet (infrabasals), and fixed ray plates ( fixed brachials above, radials below) extending downward into the cup from the free arms, and 2) large irregularly plated mid-cup and conjoining interray areas. Topologically, all plating of early crinoid crown groups is represented in the protocrinoid design. Crown-groups have mid-cup plating reduced to just one circlet or it is lacking entirely. Several problematic Paleozoic crinoids have atypical morphology that can be homologized with protocrinoid traits. Eknomocrinus, a new camerate, and Anomalocrinus, a disparid, have an infrabasal gap plate. Reteocrinus and Stipatocrinus, both camerates, have web plating above arm branchings. The monocyclic ?cladid Perittocrinus and new camerate Habrotecrinus have accessory secondary mid-cup and interbrachial plates. Cladids have additional mid-cup plating below the C-ray (anals). The Devonian cladid Belanskicrinus and Carboniferous acrocrinids both have irregular mid-cup plating. The DRHS can also be applied to other problematic crinoids including Cleiocrinus with its combined radial-basal circlet and Acolocrinus with its fixed pinnule-bearing primibrachs. The origin of an additional cup circlet found in Aethocrinus and Colpodecrinus remains problematic, but is positionally equivalent to the proximal pentamere circlet.