Revision of the Hexacrinitidae (Crinoidea) based on a classical Lower Givetian crinoid deposit (Gerolstein, Eifel/Germany)

摘要

The classic Lower Givetian crinoid occurrence of the northeastern slope of the railway cut near the station of Gerolstein (northwestern Rhineland-Palatinate, westernmost Germany) is famous for yielding an outstanding diversity of the monobathrid camerate family Hexacrinitidae. Following a short palaeogeographical and stratigraphical introduction of the Gerolstein Syncline (Eifel, Rhenish Massif), the previously described "Hexacrinites" species of this locality are revised. They clearly differ from the type species Platycrinus interscapularis (genus Hexacrinites) by the development of uniserial arms, longer aboral cups and other morphological criteria, like a single posterior interradial plate. Therefore, most of the Eifel species are assigned to the genus Megaradialocrinus (with Megaradialocrinus conicus as its type species), which is herein transferred to superfamily Hexa-crinitoidea and family Hexacrinitidae. The extent of morphological differences among other hexacrinitids is discussed and may define further intergeneric differentiation. Five new species are described: Megaradialocrinus aliculatus n. sp., (?)M bulbiformis n. sp., M. piriculaformis n. sp., M theissi n. sp. and M winteri n. sp. The homonym "Hexacrinites magnificus" sensu Hauser (2007a) is renamed: Megaradialocrinus globohirsutus nomen novum.