Cyclocystoids (Echinodermata) from the Upper Ordovician (early Katian) Kirkfield and Verulam formations of Ontario, Canada: implications for cyclocystoid skeletal homologies, anatomy, functional morphology, life mode, and systematics

New discoveries prompt reinterpretations of one of the most enigmatic echinoderm classes, the Cyclocystoidea. Exceptionally well-preserved cyclocystoids have been discovered in the Upper Ordovician (Katian) Kirkfield and Verulam formations in the Lake Simcoe region of southern Ontario, Canada. Four different cyclocystoid taxa are present, including Cyclocystoides cf. C. scammaphoris , Nicholsodiscus cf. N. anticostiensis , Zygocycloides marstoni , and Brechincycloides stanhynei new genus new species. These specimens reveal that the central disk of cyclocystoids consists largely of perforate extraxial plates and that the ever-present sutural pores are epispires surrounded by annular plates that may or may not be in lateral contact with each other. The specimens of B. stanhynei n. gen. n. sp. are significant because they are the first cyclocystoid fossils to reveal that the hydropore/gonopore and periproct are positioned on the aboral side of the central disk. The suggestively complex marginal ring, previously of undetermined origin, represents articulated floor plates in a circular arrangement supporting a circumferential water vascular system. Furthermore, the marginal ring reveals growth zones that correspond to axes of pentaradial symmetry. In the new interpretation, cupules are tube foot basins, and channels through the marginal ossicles can be explained best in the context of the axial origin as floor plates accompanied by a cover plate system. Application of the Extraxial–Axial Theory (EAT) compels a revised understanding of key anatomies comparable with other early radial echinoderms. We document evidence for cyclocystoid orientation in life with epispires facing upward and the tube foot basins in the floor plates directed towards the substrate. The new family Brechincycloididae is proposed for all cyclocystoids whose apomorphy consists of a flat, attenuated shelf tapering away from the proximal side of each floor plate. UUID: http://zoobank.org/605fdf27-b38b-4d41-824b-d10b139f90f5