Abstract
Two rudist taxa, Epidiceras speciosum (Münster) and Monopleura sp. were discovered from the allochthonous Torinosu-type limestone blocks in the Oriai Formation of the Imaidani Group in the Shirokawa area, Southwest Japan. The occurrence of Epidiceras speciosum, indicating a late Kimmeridgian to early Valanginian age, is consistent to the previous age assignment based on the occurrences of middle Kimmeridgian to early Tithonian ammonites (Hybonoticeras) from the overlying Nakatsugawa Formation and the Tithonian radiolarian assemblage of the Imaidani Group. These limestone blocks were thus originally deposited in a shallow marine shelf in the late Kimmeridgian-early Tithonian, and were transported into a deeper environment of shelf slope in the early Tithonian. Monopleura sp. from the Shirokawa area possibly represents the earliest record of this genus and family. The occurrence of Late Jurassic rudists in Torinosu-type limestone suggests that rudists had already expanded globally and that a tropical-subtropical condition prevailed in the shallow marine shelf of East Asia, where the Torinosu-type limestone was deposited at that time.
