Abstract Dinosaur eggs were recovered from the Dadaepo Basin in Busan (south‐east Korea), which is early Late Cretaceous in age and is close to Japan, but the eggs have not been studied. Among East Asian egg‐bearing basins, the Dadaepo Basin is characterized by exceptionally high thermal maturity ( c . 310°C), providing a unique opportunity to examine the influence of thermal maturation on the microstructural preservation of fossil eggs. In this study, we describe at least two distinct oogenera recovered from the Dadaepo Formation. The first is identified as Mosaicoolithus , an ootaxon indigenous to East Asia, while the second corresponds to Macroelongatoolithus , an ootaxon widely distributed across the lower Upper Cretaceous deposits of East Asia and North America. Notably, the microstructural preservation of eggshells from the Dadaepo Formation is nearly identical to that observed in specimens from less thermally mature basins in East Asia. This indicates that elevated thermal maturity does not necessarily result in the alteration of eggshell microstructure during fossilization. Microscopy and geochemistry assessments of fluid‐mediated alteration may insufficiently screen thermally matured eggshells for clumped isotope analysis, which remain susceptible to geothermal heating. The Dadaepo Macroelongatoolithus refines the global chronological range for Macroelongatoolithus to the Cenomanian–Turonian. It indirectly supports an early Late Cretaceous age for the controversial age of the Iren Dabasu (or Erlian) Formation (Inner Mongolia, China), known for giant oviraptorosaurs. Furthermore, this global age constraint highlights Cenomanian–Turonian deposits in the Bering Strait region as promising targets for future Arctic Macroelongatoolithus discoveries.
Choi et al. (Fri,) studied this question.