Fossil Isocrinida (Echinodermata) from the Miocene deposit in the Chichibu Basin, Central Japan

Abstract

For the first time, fossils Isocrinida were discovered from the Miocene deposit in the Chichibu Basin, Saitama Prefecture, Central Japan. The fossils of Isocrinida are 14 specimens from the rubbly mudstone of the upper Chichibu-machi Formation in the Chichibu Basin Group exposed on the right bank of Arakawa River, Kami-kagemori, Chichibu City, on the southern margin of the Chichibu Basin. Diameter of large columns is about 10 mm. Lateral surface of internodals are stellate. Petal is lanceolate. Cirrus sockets of nodals are always five in number. Number of internodals is 7 (sometimes 8 or 9). However, since it lacks the calyx and arms that are reliable characters for species identification, all of these specimens are assigned here as ‟Isocrinus ?” sp.. 　The fossil record of ‟Isocrinus ?” sp. have been reported from the outer shallow to central bathyal zone of the early to middle Miocene in Hiroshima, Hokuriku and Gunma in Japan, suggesting the sedimentary environment of the Chichibu-machi Formation to be similar depth.