Revision of Scoliocystis (Rhombifera: Echinoencrinitidae) and related cystoid genera

Abstract

Species attributed to Scoliocystis Jaekel, 1899, including the type species S. pumila (Eichwald, 1860) and S. thersites Jaekel, 1899 from the Ordovician of the Leningrad Region, are reviewed. Scoliocystis sp. from the Upper Ordovician of Estonia, figured by Hecker (1964), is redescribed as Maennilocystis heckeri gen. et sp. nov. and it is attributed to the family Callocystitidae Bernard, 1895. The genus Scoliocystis is inter mediate between the families Cheirocrinidae and Echinoencrinitidae in having five periproct border plates (as in cheirocrinids), but lacking plate R5 and having a reduced oral area, as in echinoencrinitids, but is retained in the Echinoencrinitidae. The similarity of Scoliocystis to the unusual North American cheirocrinid genus Sprinkleocystis is convergent. The genus Gonocrinites Eichwald, 1840, with two species, is restored among echinoencrinitids, as having four periproct border plates, whereas Echinoencrinites sensu stricto has three. The families Echinoencrinitidae and Callocystitidae arose from cheirocrinids with closed plate circlets by the loss of plate R5 and substitution of radial:lateral for radial:radial pectinirhombs. In addition, echinoen crinitids are characterized by a reduced oral area, whereas in callocystitids, the recumbent ambulacra are extensive. Scoliocystis is a stem group genus to Echinoencrinitidae plus Callocystitidae. Several glyptocysti toid genera independently developed advanced pectinirhombs.

Primitive stalked echinoderms from the Middle Ordovician (Darriwilian) of Bang Song Tho, Kanchanaburi, western Thailand

“…Diagnosis . A dichoporite superfamily with well-developed, xenomorphic stem consisting of a rapidly tapering proximal portion composed of alternating inner and outer annular columnals, and a distal portion composed of alternating longer and shorter cylindrical columnals; theca composed of four basals, five infra-laterals, five laterals, four, five, six or ten radials, and seven orals (after Paul & Rozhnov, 2016, p. 260).…”