Fossil eggshells of amniotes as a paleothermometry tool

Raman spectroscopic analyses of thermally altered organic materials can be used to assess the paleothermometry of the sedimentary deposits. Although this technique has been widely applied to diverse microfossils, macroscopic vertebrate fossils have been neglected. In this paper, we show that fossil eggshells can be used for this purpose by demonstrating the paleothermometric potential of diverse amniotic eggshells from the Wido Volcanics (Upper Cretaceous, South Korea) that contain thermally altered organic material. We estimate the maximum paleotemperature recorded in the eggshells using Raman spectroscopic data and the spectrum deconvolution technique, which was invented and developed by organic geochemists. The results show that the same type of eggshells record different paleotemperature gradients depending on their spatial distribution in the fossil locality, whereas different types of eggshells from similar locations show similar paleotemperature gradients (except for one specimen with a peculiar microstructure). These findings are further supported by Fourier transform infrared spectroscopic results, which focus on thermally altered organic materials, and electron backscatter diffraction data, which focus on calcite twinning. This study suggests that fossil eggshells are useful and reliable materials for paleothermometry because thermally altered organic materials and calcites of eggshells provide independent opportunities to assess paleothermometry. We propose that fossil eggshells may be useful for evaluating the thermal evolution of sedimentary basins.