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Lithofacies and biofacies of a rudist-bearing reefal limestone from the uppermost Jurassic Tsui Formation on the Youra Peninsula, eastern Oita Prefecture, southwest Japan, were quantitatively examined. The 35-m-thick limestone section displays a vertical transition from a reefal fauna in the lower–middle part (dominated by hermatypic corals and stromatoporoids) to the upper part (characterized by abundant rudists, corals, and chaetetids). Lithological features and faunal compositions indicate an oligotrophic shallow-water depositional environment in a calm setting on a small carbonate mound. In the upper part of the section, requieniidine rudists (Epidiceras speciosum) occur densely in layers 0.5–1.5 m thick, forming biostromes with corals. Reconstructed ecological features of Late Jurassic Epidiceras are similar to those of Triassic megalotondntid and Cretaceous requieniid rudists, but differ greatly from those of primitive uncoiled hippuritidine rudists ( ‘Valletia’ cf. auris) reported from another Torinosu-type limestone in the Shirokawa area, Shikoku Island, southwest Japan (Upper Jurassic–lowermost Cretaceous). These results suggest that rudists had already ecologically diverged in the late Late Jurassic, during an early stage of their evolutionary history.

大分県東部・四浦半島に分布する後期ジュラ紀の礁性石灰岩の堆積環境と厚歯二枚貝の産状