U-Pb calcite age dating of fossil eggshell as an accurate deep time geochronometer

Earth’s sedimentary rock record is the primary archive for biotic, environmental, and climatic trends in deep time. Reconstructing these patterns requires a high-resolution geochronologic framework. This remains a significant challenge for many terrestrial ecosystems and an acute problem for some of the world’s most important Mesozoic and Cenozoic fossil records. Overcoming this issue requires frontier approaches, such as directly dating fossils, long considered untenable. Here, we test the reliability of novel LA-ICP-MS U-Pb calcite dating and elemental mapping of non-avian dinosaur eggshells to produce accurate “burial ages.” We directly dated fossilized dinosaur eggs recovered from the Western Interior Basin of North America, producing ages within 5% of high-precision ages from bracketing ash beds. We then directly dated dinosaur eggs from Upper Cretaceous strata within Mongolia’s famous yet poorly age-constrained Gobi Basin, providing the first radioisotopic age for these deposits. Geochemical data coupled with trace elemental mapping indicate early uptake of uranium (U) in non-avian dinosaur eggshells via sediment contact, consistent with findings from Quaternary avian eggs. Calcified eggs, having evolved over 250 million years ago, offers a promising experimental methodology for determining the age of globally distributed fossil assemblages and recovering temporal, environmental, and ecological data from a single fossil. Biogenic carbonate of eggshells can be used as a new geochronometer through direct calcite U-Pb dating, producing high accurate ages, as revealed by two tests on dinosaur eggs from North America and Mongolia.