2021 Volume 72 Issue 6 Pages 479-493
Modern freshwater planktonic diatom assemblages appeared near the middle/late Miocene boundary when the nonmarine Actinocyclus was replaced by the genera of the family Stephanodiscaceae. The genus Mesodictyon was a frontier that first entered the lake environment in this turnover among the family Stephanodiscaceae. In this study, Mesodictyon japonicum Yanagisawa & H.Tanaka, an important constituent of late Miocene lacustrine planktonic diatom assemblage in Japan, is reported from the Neogene marine deposits of the Shitazaki and Kubo formations distributed in the Ninohe City (Iwate Prefecture) and Sannohe Town (Aomori Prefecture). The first occurrence of this species was recognized in the lower part of the diatom zone NPD6B (Thalassionema schraderi Zone) and was dated at ca. 8.3 Ma. Fossil evidence from this study and previous researches in Japan indicates that the first occurrence of M. japonicum in Japan is confined within relatively limited time range of 8.2–8.7 Ma, implying possible isochroneity of this diatom biohorizon. Mesodictyon japonicum seems to appear in geological record at around 8.6–8.7 Ma near the base of the NPD6B.
