Three Ordovician cephalopods from the Jigunsan Formation of Korea

抄録

Cephalopod species previously described from the Middle Ordovician Jigunsan Formation of Duwibong type Joseon Supergroup of Baegunsan Syncline, Kangweondo in Korea were re-examined taxonomically based on the type and figured specimens and newly collected ones. Three species, Holmiceras coreanicum (Kobayashi, 1927), Sactorthoceras makkolense (Kobayashi, 1927), and Kotoceras grabaui (Kobayashi, 1927) are described. Each species was restudied and compared with closely related species described by Kobayashi (1927, 1934); respective lectotypes are also designated herein. In this study, Sigmorthoceras coreanicum, based on its sigmoidally curved conch, was identified as belonging to Holmiceras which is characterized by the early loosely gyroceraconic shell portion. Sinuitopsis kochiriensis, previously identified as an Ordovician gastropod, is a juvenile shell of H. coreanicum, judging by the number of the volutions and prominent shell surface. Three species belonging to Sactorthoceras and also Cycloceras taihakuense were regarded, based on the existence or nonexistence of the preserved shell and the imploded internal structure, as junior synonyms of Sactorthoceras makkolense in a broad sense. Kotoceras frechi was rejected as an invalid taxon, since its septal angulation, broader siphuncle and rapid expansion of the conch are characteristics caused by secondary deformation.