Mesodictyon japonicum Yanagisawa & H. Tanaka is an extinct centric diatom described and reported from upper Miocene marine sediments. The species had been presumed to be a freshwater species because of the occurrence pattern similar to that of accompanying Aulacoseira species which may be transported from inland lakes to marine environments. However, the possibility that this species was a marine species can not be completely ruled out. In the course of continued study, we have found the mass occurrence of M. japonicum with other freshwater diatoms in the lacustrine deposits of the late Miocene Takatama Caldera (8–9 Ma in age) in Koriyama City, Fukushima Prefecture, and thus confirmed that this species was a freshwater diatom.
2016 The Japanese Society of Diatomology