Thermal maturity and colors of Cretaceous East Asian fossil eggs

Several factors influence the quality of fossil preservation, with temperature being one of the key variables. The maximum temperature that fossils have experienced during their taphonomic history affects their thermal maturity and color. Cretaceous fossil eggs of amniote vertebrates from East Asia were analyzed using Raman spectroscopy to investigate their thermal maturity. Eggs from inland regions of East Asia (Mongolia and inland China) do not show a significant thermal imprint, while eggs from the southern and eastern regions of East Asia (coastal China, Korea, and Japan) show a high thermal maturity. Although this pattern must have been caused by the combined effect of complex factors (e.g., burial depth, stratigraphic position, and local geothermal phenomenon), the rollback of the Paleo-Pacific Plate during the Cretaceous and consequent vigorous igneous activity and/or high heat flow near the subduction boundary may have been one of the main working factors. The approach of this study can be extended to fossil eggs from other continents to elucidate the relationship between thermal maturity and geological setting, and will provide a deeper understanding of fossil egg taphonomy.