The species Gephyrocrinus grimaldii Kœhler & Bather, 1902 (Echinodermata, Crinoidea, Hyocrinidae) is revised using additional information on arm and pinnule characters, ontogeny and intraspecific variation. The validity of the monospecific genus Gephyrocrinus is confirmed and its affinities are clarified and documented. It is known in northeastern Atlantic only at depths ranging from 1420 to 1968 m. Among hyocrinids, Gephyrocrinus belongs to the group of genera including Calamocrinus, Dumetocrinus, Feracrinus and Ptilocrinus which all bear the first pinnule on the fourth brachial. Gephyrocrinus grimaldii is remarkable in having a constant pattern of regularly alternating muscular (synarthries) and ligamentary (synostoses) articulations along the middle and distal arm, and small irregular lateral plates all along the pinnules. The proximal inflation of the genital pinnules in Gephyrocrinus is built with numerous lateral plates never in rows as in Calamocrinus and Ptilocrinus, unlike the H-shaped plates characteristic of several hyocrinid genera such as Dumetocrinus, Feracrinus and Hyocrinus. Long series of successive brachial pairs are also frequent among the genus Hyocrinus. Here, we interpret this character as an adaptive derived character independently appearing in different clades rather than as a synapomorphy. Arm branching occurs exceptionally at the fourth brachial in Gephyrocrinus and Dumetocrinus, whereas it never appears before the eighth brachial in Calamocrinus. Using pinnule architecture as the most discriminating character, Gephyrocrinus appears to have the closest affinities with Calamocrinus and Ptilocrinus.
Echinodermata, Crinoidea, stalked crinoids, Hyocrinidae, Calamocrinus, Gephyrocrinus, Hyocrinus, Ptilocrinus.