DIPLOBATHRID AND DISPARID CRINOIDS FROM THE MIDDLE ORDOVICIAN JIGUNSAN FORMATION, SOUTH KOREA: THEIR FIRST RECORD FROM THE SINO-KOREAN PLATFORM

Eleven crinoid specimens are found in dark greenish to bluish gray laminated shale slabs of the Middle Ordovician Jigunsan Formation, South Korea. The slabs were collected near the outcrops of the formation and contain disarticulated sclerites of trilobites typical of the formation such as Dolerobasilicus. Most crinoid specimens have cups, arms and columns preserved as external molds. Taxonomic study in progress indicates that one specimen belongs to the Diplobathrida and the remaining ten, which all occur in a single slab, to the Disparida.

The diplobathrid specimen is characterized by having a sub-globular cup with strong stellate and median ray ridges, expanded cup base with circular ridges and concave circlets, uniserial and pinnulate arms, and rounded column with a circular lumen. These features indicate that the specimen can be assigned to Pararchaeocrinus of the family Rhodocrinitidae.

Of ten disparid specimens, three for which the cups are well preserved can be classified into three different genera. The specimens share a monocyclic cup which is small-sized relative to column and isotomously branching uniserial arms; the first feature is diagnostic to the Disparida. The first specimen assignable to Heterocrinus of the Heterocrinidae is characterized by having a fused radial plate and separate superradial, infraradial and anal plates, and straight lower margin of basal plates. The second assignable to Vosekocrinus of the Maennilicrinidae shows a cuspate mid-point of lower margin of basal plates and obtusely pentalobate column, and lacks a radial plate. The third attributable to Iocrinus of the Iocrinidae has a straight lower margin of basal plates, and pentagonal column. Detailed taxonomic evaluations of these specimens and the remaining seven specimens for which the cups are relatively poorly preserved are pending.

The diplobathrids have been reported from the Middle Ordovician strata of North America, Spain and Wales, and the disparids from those of North America, Oman, South China and Morocco. This is the first report of the Middle Ordovician diplobathrid and disparid crinoids from the Sino-Korean platform (North China platform). In addition, it appears to be intriguing that the Jigunsan crinoid fauna, in particular the disparids, apparently shows a high taxonomic diversity at the family level.