Morphological Reevaluation and Revision of the Cleiocrinidae S. A. Miller 1890

Cleiocrinus Billings 1857 is unquestionably one of the most enigmatic camerate crinoids. Diagnostic characters include: (1) a ten plate cup-base circlet composed of alternating basals and radials that overgrow hidden infrabasals; (2) absence of interray plates except for a single column of anal plates; and (3) abundant pores on most cup-plate sutures. Cleiocrinus is presently known from the Middle-Late Ordovician (Whiterockian-Mohawkian) of Laurentia and includes ten named species, but it is likely that several of these should be synonymized.

Reevaluation of newly discovered and museum specimens reveals that greater morphological and genus diversity exists in the family than previously recognized. This includes two new genera: (1) several specimens from the Bobcaygeon Formation (Ontario) that have equant, pentagonal cup-base plates with radials that form a W-shaped suture with overlying primibrachs, and (2) a specimen from the Bromide Formation (Oklahoma) with radials fused with first primibrachs, three primibrachs total, and relatively few interplate pores.

The tegmen of Cleiocrinus is not exposed in any known calyces. However, Astroporites ottawaensis Lambe 1894, first identified as a bryozoan, and subsequently interpreted as a large echinoderm holdfast, is here considered the tegmen of Cleiocrinus. Astroporites is a flattened disklike structure with numerous pores. Morphological characteristics that support its interpretation as a cleiocrinid tegmen include the combination of: (1) echinoderm calcite and pentameral symmetry; (2) a branching ambulacral system; (3) an anal vent; (4) numerous small plates that are mostly fused and riddled with pores; and (5) their association with disarticulated Cleiocrinus plates and arms. The fused nature of the Cleiocrinus tegmen often results in its exceptional preservation as a single disk, even though the dorsal cup with its numerous interplate pores and relatively weak plate sutures readily disarticulates.