Turtle eggs from the Lower Cretaceous Hasandong Formation (South Korea) with relict aragonite under significant thermal maturity

Unlike the eggshells of other amniotes, turtle eggshells are composed of aragonite, which is a metastable mineral. Turtle eggshells in the fossil record are therefore usually transformed to calcite. Geothermal heat also negatively affects the preservation of aragonite, and therefore the preservation of aragonite under geothermal settings is not usually expected. Here we report new turtle eggs from the Lower Cretaceous (Aptian-Albian) Hasandong Formation of South Korea, describing the morphological features of the eggs and eggshells. The eggs belong to an oogenus Testudoolithus with the smallest recorded egg size. As one of the oldest fossil eggs from Korea, this material fills a paleobiogeographic gap in the fossil record of turtle eggs in East Asia, which has limited records for the eggs of non-dinosaurian reptiles. More importantly, the presence of relict aragonite was cross-validated by electron backscatter diffraction and Raman spectroscopy. The Raman spectra of the thermally altered organic matter inside the eggshells indicated that the eggs experienced a maximum temperature of almost 260 degrees C during their taphonomic history. This implies that aragonite can be preserved even under hostile thermal conditions and earlier reports of 'calcite-only' turtle fossil eggs may preserve undetected relict aragonite, which is only detectable via careful investigation using advanced microscopic techniques. The combined use of mineralogical and spectroscopic approaches adopted in this study may also be useful to invertebrate paleontology and archeology to further understand the relationship between the preservation of aragonite and the maximum paleotemperature that the materials experienced.