2025 Volume 79 Issue 2 Pages 67-80
Various well-preserved fish scale fossils were found in laminated black shale from 11 Lower to Middle Miocene Josoji Formation sites on the Shimane Peninsula in southwestern Japan. Based on the observed morphologies, five families – Clupeidae, Myctophidae, Halosauridae, Sparidae, and Scombropidae – were identified for the first time in this study. To reconstruct the depositional paleoenvironment of the study sites, analyses of fossil benthic foraminifera and total organic carbon, nitrogen, and sulfur contents were also conducted. The sea bottom paleoenvironment of the lower part of the Josoji Formation was highly reductive with oxygen-deficient water masses, allowing only neritopelagic clupeid fish scales to be preserved at the sea bottom. During the deposition of the lower to middle part, the paleoenvironment shifted to a slightly oxidative sea bottom in the middle bathyal zone, enabling mesopelagic myctophid fish and demersal halosaurid fish to live in deep seas. In the upper part, the paleoenvironment transitioned to an oxygenated sea bottom in the middle bathyal zone, resulting in a high diversity of fish fauna. Based on the correlation with the paleoenvironments of other contemporaneous formations in the coastal area of Japan Sea, transgression and deepening are inferred to have occurred earlier at the study site.
