Non‐Marine Diatom Biostratigraphy Based on Evolutionary Changes of the Genus Praestephanos in the Pliocene Lacustrine Deposits of the Kameyama Formation, Tokai Group, and Its Correlation With the Kobiwako Group, Central Japan

To gain new insights into non‐marine diatom biostratigraphy, we provided taxonomic descriptions and examined the morphological evolution of the freshwater diatom genus Praestephanos from the Pliocene lacustrine deposits of the Kameyama Formation, Tokai Group in Mie Prefecture, Japan, which were deposited in Paleo‐lake Anoutsu‐ko. We also compared these results with the evolutionary record of Praestephanos from the contemporaneous lacustrine deposits of the Ueno Formation, Kobiwako Group, reported in a previous study, to establish a biostratigraphic correlation between the two formations; the deposits of the Ueno Formation accumulated in Paleo‐lake Oyamada, which is thought to have been hydrologically connected to Paleo‐lake Anoutsu‐ko. In the Kameyama Formation, P. tokaiensis and P. miensis , originally described from the Ueno Formation, occur in intervals of approximately 3.9–3.8 Ma and 3.8–3.6 Ma, respectively. Around 3.8 Ma, during the transition between the two species, changes in the number of valve face fultoportulae were observed, suggesting a morphological shift from the ancestral P. tokaiensis to the descendant P. miensis . This stratigraphic distribution and morphological evolution were consistent with patterns observed in the Ueno Formation, indicating that the same evolutionary changes in Praestephanos occurred isochronously in both lakes. Therefore, the species turnover horizon can be regarded as a biostratigraphic datum for both the Kameyama and Ueno Formations. Notably, the two species were likely endemic to this paleo‐water system. Although the biogeographic provinciality of freshwater diatoms can often be a limitation in biostratigraphic research, the results of this study demonstrate that biostratigraphic correlation based on evolutionary changes is possible between formations in regions belonging to the same paleo‐water system, even for freshwater diatoms with strong provinciality. Furthermore, the finding that the same diatom species exhibited isochronous morphological evolution with an identical pattern in both regions within the same paleo‐water system suggests that comparative analyses of diatom evolutionary changes and stratigraphic distributions between areas can potentially be developed into indicators for reconstructing paleo‐water systems.