THE TEGMEN AND PORE-PLATE SYSTEM OF CLEIOCRINUS

We have acquired, through field work and other acquisitions, specimens of Cleiocrinus Billings 1857 (Crinoidea; Camerata) that confirm the nature of its tegmen and pore-plate system. Astroporites ottawaensis Lambe 1894, currently classified as a bryozoan, is conclusively the tegmen of Cleiocrinus. We possess numerous specimens in which Astroporites and Cleiocrinus are cojoined. This permits the first definitive description of the Cleiocrinus tegmen and its relationship to the rest of the calyx. It also increases its standing as one of the most bizarre of Paleozoic crinoids. The genus is known only from the Middle to Late Ordovician of North America; the new specimens are from (Katian) strata near Brechin, Ontario, Canada.

The tegmen of Cleiocrinus is composed almost entirely of ankylosed brachial and pinnular cover plates, all united in a rigid disk. Between each of the relict cover plates is a small pore. Larger pores are developed along the junctions of adjacent series of cover plates. This is best observed from the interior surface of the tegmen, as the exterior surface bears a mosaic of tiny, chaotically arranged plates. Also visible on the interior surface is a centrally positioned, hexagonal peristome divided into five coelomic spaces that are separated by thin, interradially arranged plates, possibly orals. As is typical of camerates, Cleiocrinus has a subtegminal mouth. An oblong ring of plates, the periproct, is positioned immediately adjacent to the peristome in the posterior (CD) interray. It is expressed on the exterior surface as a slight depression.

During ontogeny, brachials and pinnulars are fixed into the calyx at the cup-tegmen junction. As this happens, they decouple from their corresponding cover plates, which fuse along the periphery of the disc. This results in a branching pattern in the calyx that is mirrored by that of the tegmen. Moreover, as pinnulars are fused into the dorsal cup, pores develop at the plate junctions between individual pinnulars and brachials. This is the origin of the large thecal pores that are diagnostic for some species of the genus. These pores remain open during ontogeny but are gradually masked by rhomb-shaped grates. Such changes in ontogeny, in addition to differences generated by taphonomy and weathering, have likely resulted in an inflated number Cleiocrinus species.