FOSSILIZATION OF WELL-PRESERVED DEEP-SEA LANTERNFISH: A CASE STUDY FROM THE MIOCENE MOROZAKI GROUP, CHITA PENINSULA, SOUTHWEST JAPAN

Abstract Several biological and chemical changes occur during fossilization of organisms. However, the nature and mechanism of fossilization are not yet completely understood. In this study we investigated the changes in chemical composition during fossilization of lanternfish from the lower Miocene Yamami Formation, Chita Peninsula, Japan compared to modern/living lanternfish. This comprised chemical mapping using X-ray microscopic analysis, mineral identification using Raman spectroscopy, and chemical analysis using inductively coupled plasma mass spectrometry and elemental analysis. Carbon and nitrogen were lost significantly during fossilization, whereas slight changes were observed in phosphorus and calcium concentrations, which are major elements of hard tissues. Iron and sulfur concentrations are high in fossil fish compared to modern/living fish due to pyrite formation during fossilization. In agreement with earlier studies, we conclude that in-situ pyrite formation mediated by sulfur-reducing bacteria played an important role in the preservation of soft tissue textures in the Yamami Formation. This includes the preservation in the fossils of delicate organs, such as eyes in the Yamami lanternfish. The oxidation of pyrite is also important for fossil preservation because of the low solubility of iron oxides.