NEW DATA ON A LATE DEVONIAN (FRASNIAN) CYCLOCYSTOID FROM THE ITHICA FORMATION OF NEW YORK STATE

Cyclocystoids are a clade of discoidal echinoderms with a well-developed marginal ring and multiply branched ambulacra that were uncommon components of Lower and Middle Paleozoic faunas. Cyclocystoids were rare by the Frasnian (Late Devonian), and none are described from rocks younger the Frasnian/Famennian Mass Extinction. Here we report on two specimens of a new species of the cyclocystoid Polytryphocycloides from the Late Devonian (Frasnian) Ithica Formation, Genesee Group from New York, USA. Both of these specimens are preserved as external molds in siltstone of the ventral surface of the test. Consequently, they were studied primarily utilizing latex casts of the natural molds. Preserved specimens of Polytryphocycloides n. sp. are up to 80 millimeters in diameter making this the largest species of cyclocystoid known. This species is also the youngest known member of the genus, extending the North American stratigraphic range of Cyclocystoidea into the Upper Devonian.

Polytryphocycloides n. sp. is characterized by a ring of heavy marginal ossicles with two (rarely one) cupules and crests that display a unique ornamentation with subparallel rugae along the median of the crest that fan/splay laterally towards lateral striae. One specimen preserved with a complete marginal ring bears 49 marginal ossicles. The ventral surface of these plates is strongly concave as well as both the proximal and distal margins, giving it a slightly waisted appearance. In lateral view the crests are convex giving them a semi-elliptical cross section. Plates bearing two cupules are nearly twice as wide as those bearing one cupule and single cupule plates are inserted into the margin at random. The cupules are slightly longer than broad with low lateral walls that are absent proximally and distally and each bears a rounded tubercle that is slightly longer than broad. The smooth frontal plates are as tall as they are broad and are more curved ventrally than distally. Roofing plates are distally pointed and form five to six alternating rows. Imbricate radial plates are stellate in shape with two lateral processes and a shallow radial channel. The lateral processes from adjacent rays abut along their lateral margins.