Apatite U-Pb Age of the Baynshire Formation, Gobi Desert, Mongolia, Using Fossil Teeth

Nishimura, R., Aoki, K., Kogiso, T., Saneyoshi, M., Buyantegsh, B., Mainbayar, B., Tsogtbaatar, K., Chiba, K.

The 6th International Symposium on Asian Dinosaurs in Japan 2025

Asian Dinoaur Association

Fukui, Japan

The Upper Cretaceous strata of the Gobi Desert in Mongolia are internationally renowned for yielding abundant vertebrate fossil remains, particularly those of dinosaurs. However, the exposures of these strata are generally thin and discontinuous over a wide area, with the scarcity of key index fossils and volcanic ash layers that are typically useful for precise age determination. As a result, their stratigraphic relationships and depositional ages remain subjects of ongoing debate. Recent advances in geochronology offer a solution: in situ U‑Pb dating of apatite comprising teeth can constrain depositional ages because uranium can be incorporated during fossilization, and the uptake is suggested to occur on timescales shorter than analytical uncertainties. We conducted U-Pb dating on dinosaur and crocodilian teeth from Bayn Shire, Shine Us Khuduk II, Bayshin Tsav, and Urlibe Khuduk localities, where the Baynshire Formation is thought to be exposed. The overall depositional age of the Baynshire Formation is suggested to range from Cenomanian to Santonian (approximately 101 to 84 Ma) based on K-Ar dating of the interbedded basalt, U-Pb dating on caliches, freshwater mollusks, and turtle fossils. Although there is a general consensus on the overall depositional age of the formation, determining the depositional age at each fossil locality is necessary due to the long depositional age of the formation to temporally correlate each locality and construct the temporal framework of the formation. To identify the least-diagenetically altered samples for U-Pb dating, we conducted geochemical screening based on the yttrium (Y) concentration mapping using micro‑XRF and trace element analysis using LA-ICP-MS. The mapping and trace element analyses reveal that all specimens display high, uniform Y concentrations. Similarly, the normalized rare earth element patterns are similar across all samples regardless of their stratigraphic levels or collection localities, suggesting the influence of diagenetic alteration.  Although we cannot eliminate the influence of diagenesis, we conducted U-Pb dating on three specimens, which have a relatively heterogeneous distribution with low Y concentrations. Although the U-Pb age of two of the three samples was calculated as approximately 50 to 30 Ma, a crocodilian tooth from Urlibe Khuduk yielded a U-Pb age of approximately 90 Ma (close to the age of the Turonian/Coniacian boundary). The Y-mapping and trace element analysis both indicate the age resetting of the examined samples. Nonetheless, the Urlibe Khuduk sample suggests that the depositional age of the sequence exposed at the site is younger than the latest Turonian. The result here is congruent with the previous biostratigraphical correlation of the locality to the Cenomanian to Turonian "lower" Baynshire Formation. Recent excavations have revealed abundant dinosaur fauna at the locality, of which paleontological importance will be further enhanced by the absolute dating reported here. This study highlights the importance of non-traditional radiometric dating on calcites and apatites in the Late Cretaceous Mongolian strata, which will lead to the construction of a better temporal framework.