Revision of the class Cyclocystoidea (Echinodermata)

Abstract

Almost all known cyclocystoid specimens have been examined and the class revised. Cyclocystoids are very distinctive and consist of a complexly plated disc surrounded by a marginal ring of stout, perforate, cupule-bearing ossicles, in turn encircled by a narrow, flexible, plated peripheral skirt. The disc consists of: (1) a ventral surface with branched rays bearing irregular, immovable cover plates over narrow channels and unbranched interrays (except in Actinodiscus nov.); both uniserial and composed of imbricate plates; and (2) a dorsal surface usually covered by polygonal, annular plates. Sutural pores occur between radial and interradial plates ventrally and align exactly with the central perforations of the dorsal annular plates. A ventral mouth and dorsal anus occur subcentrally in the disc. The marginal ring consists of large ossicles each with a ventral crest and one to seven cupules. Marginals articulated with each other laterally and were perforated by two types of canal. The peripheral skirt consists of larger frontal plates, one per cupule, and smaller imbricating roofing plates. It was flexible and could cover the entire cupule zone of the marginal ring. The cyclocystoid test resembled a tambourine, not a drum. Space for internal anatomy was extremely limited. The gut was straight and very short. Microphagous organic feeding was the only plausible method. We believe that food was gathered in the cupules, passed through the radial ducts of the marginals into covered radial channels of the disc and thence to the mouth. Locomotory tube feet were housed in the ventral sutural pores. We believe that cyclocystoids lived with the oral surface below, were mobile and gathered food from the sediment surface. The test grew in a complex fashion. Disc elements were added peripherally throughout growth. Marginal ossicles were added very quickly early in growth and the number remained fairly stable thereafter, but cupules and radial ducts continued to be added so that the number per ossicle increased. Cyclocystoids appeared in the Middle Ordovician of North America and are last recorded from the Middle Devonian of Europe. They were originally relatively rare but much more diverse than previously suspected. Analysis of gaps suggests that the fossil record of cyclocystoids at generic level is at least 36% incomplete. The class consists of a single family, the Cyclocystoididae, characterized by cupulebearing, perforate marginals and branched radii. Species with imperforate marginals (including the Middle Cambrian ‘ C .’ primotica Henderson & Shergold) are removed from the class. As restricted, the family includes the following genera: Cyclocystoides Salter & Billings, 1858; Narrawayella Foerste, 1920; Actinodiscus nov.; Apycnodiscus nov.; Diastocycloides nov.; Polytryphocycloides nov.; Sievertsia nov. and Zygocycloides nov. Genera are distinguished on the marginal ossicles (which may be in contact or separated dorsally and may or may not have cupular tubercles) and on the presence or absence of interradii or dorsal interseptal plates. We accept 25 species, plus five left under open nomenclature. New species are Cyclocystoides latus, C. scammaphoris, C. tholicos, Diastocycloides stauromorphos, Polytryphocycloides grandis, Sievertsia , Zygocycloides marstoni (Salter ms.) and Z. variabilis .