Abstract
Fifty molluscan species have been recovered from boulders in the lower Middle Miocene Sazen Formation at Unazukimachi-Oritate in Kurobe City, Toyama Prefecture. The assemblage includes new species of the trochid Protorotella and the plesiotrochid Trochocerithium, herein described. Based on the living species of the same genera, it can be estimated that the Oritate assemblage was deposited in water deeper than 50 m. Some characteristic species of Yatsuo-Kadonosawa and Shiobara-Yama faunas are mixed in the assemblage. Moreover, some cold-water species are shared with the Early Miocene Sankebetsu and the Middle Miocene Chikubetsu faunas in Hokkaido which first appeared in the Japan Sea borderland of Honshu. The first appearance of cold-water species corresponded with the Middle Miocene Climatic Transition (MMCT) around oxygen isotope zone Mi 2a.
https://ror.org/04r8tsy16 
