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

Аннотация

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 intermediate 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, echinoencrinitids 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 glyptocystitoid genera independently developed advanced pectinirhombs.