Thirty years ago, thousands of fossilized dinosaur eggs were discovered in Yunyang District in central China. Now, for the first time, researchers have directly dated the preserved shells, using a method that could be applied to other carbonate-based fossils around the world (Front. Earth Sci. 2025, DOI: 10.3389/feart.2025.1638838).
Typically, archaeologists determine the age of a fossilized dinosaur egg using indirect methods, like dating the rock or ash around it or comparing other fossils in the egg’s strata to global stratigraphic data. These methods, however, are prone to indirect errors because of later volcanic eruptions or other geological activity.
By dating the fossilized eggshell directly, researchers can determine a more accurate depositional age. The trick, as Bi Zhao, a researcher at the Hubei Institute of Geosciences who was part of the research team, explains via email, “was that the method we used—uranium-lead dating of carbonate materials— hadn’t been applied to fossil eggshells before.”
The team sampled both the fossilized eggshell and the calcite that fills the egg and then analyzed the samples using a laser ablation multicollector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS). The researchers found that the eggshell, which is composed of a mixture of secondary calcite and primary carbonate minerals, contained more uranium and radiogenic lead than the calcite egg filling. “During the experiment, we discovered that the eggshell calcite itself yielded more consistent results than the infill calcite we had originally intended to date,” Zhao says, describing it as a “real surprise.”
But it wasn’t just the results that surprised Zhao. “The first thing I thought when I saw the egg up close was, ‘How did the baby dinosaur ever break out of such a thick shell?!’ ” he writes.
Analyzing the biogenic calcite from the thick eggshell led the researchers to settle on a depositional age of 85.91 million years ago for the dinosaur eggs, with an uncertainty of ± 1.74 million years. That puts the eggs as being laid during the late Cretaceous period.
“The Cretaceous was a period characterized by major environmental changes, including global warming, oceanic anoxic events, and mass extinctions. In general, compared to marine organisms, the responses of terrestrial organisms to such environmental perturbations remain less well understood,” Kohei Tanaka, who researches stratigraphy and paleontology at the University of Tsukuba, says in an email. “If the depositional age of fossil eggshells can be accurately determined, it becomes possible to examine their relationships with major environmental changes of the Cretaceous and to compare them with fossil assemblages from other regions.”
The applications of this new method are not limited by location or organism. Both Zhao and Tanaka say the approach could be applied worldwide and on nondinosaur eggs. “By building precise timelines of dinosaur egg fossils from around the world, and integrating this data with other paleontological and geological evidence, we may contribute to a clearer picture of dinosaur evolution—and extinction,” Zhao writes.
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