T he Devonian crinoid genus StylocrinusSandberger and Sandberger, 1856, is characterised by relatively simple crown construction with eight plates within the monocyclic aboral cup (three basals and five radials) followed by five atomous arms. Isolated aboral cups have been reported from Europe (e.g. Goldfuss 1839; Sandberger and Sandberger 1849–1856; Müllerin Zeiler and Wirtgen 1855; Schultze 1866), Asia (Reed 1908; Dubatolova 1971) and Australia (Jell and Jell 1999). Schultze (1866) and Sandberger and Sandberger (1849–1856) also described partly preserved crowns from Germany, where the genus is most abundant within the Eifelian and Givetian deposits of the Rhenish Massif (Eifel and Lahn-Dill vicinity). The Eifel Synclines contain the most famous localities of this low diversity but highly variable crinoid genus.
Since the nineteenth century, stylocrinids have been assigned to several genera, including Platycrinites (Goldfuss 1839), Hexacrinus (Reed 1908), Symbathocrinus (Müllerin Zeiler and Wirtgen 1855) or Scytalocrinus (Wachsmuth and Springer 1886). But the simple construction of the aboral cup, defined by Goldfuss (1839, p. 345) and Sandberger and Sandberger (1856, pp. 399–400), allows an unquestioned identification, with the exception of the arms, which were incorrectly described as ‘additional, elongated radials’ by Sandberger and Sandberger. Following the revised diagnosis given herein, three species are recognised, S. tabulatus (Goldfuss, 1839), S. granulatusHauser, 1997, and S. prescheri sp. nov., based on differences in the skeletal features and plate sculpturing.
Stylocrinus tabulatus has high ecophenotypic plasticity expressed as morphological variability of the aboral cup. The length and width proportions of c. 1500 aboral cups have been analysed and interpreted. As a result, S. tabulatus altus and S. t. depressus both (Müllerin Zeiler and Wirtgen, 1855) are rejected. Therefore, the subspecies S. tabulatus tabulatus is redundant.
Rare crowns (Text-figs 1, 2A–C, E–F) and several isolated brachials (Text-fig. 3A–N) represent an extraordinary construction of the atomous stylocrinid arms. They have internally inclined edges adjoining laterally with adjacent brachials in an interlocking network and an apparently rudimentary pinnulation (Text-fig. 3C, E), arising from irregularly arranged lateral notches, which are diagonally positioned to each other. This construction potentially affords feeding without totally opening the arms in an unprotected position.
Morphologic details of stylocrinids require further research. One fossil aboral cup of S. tabulatus represents the first evidence of the radular grazing ichnogenus RadulichnusVoigt, 1977, on a crinoid cup (Text-fig. 4). Isolated radial and basal plates have post-mortem borings of unknown organisms (Text-fig. 5A–T). Furthermore, other skeletal modifications, such as an aboral cup with an additional fourth basal plate (Text-fig. 6A–B) and the post-mortem skeletal encrusting by a rugose coral (Text-fig. 7A–B), are discussed.
A neotype is proposed for the lost holotype of S. tabulatus.