A cluster of 28 fossilized dinosaur eggs discovered in the Qinglongshan fossil reserve in central China has been dated to around 86 million years old.
Scientists determined the age using a highly precise “atomic clock” approach, and they hope both the eggs and the dating technique will shed light on how dinosaurs in China’s Yunyang Basin adapted to a cooling climate during the Cretaceous period.
The method applied, known as uranium-lead (U-Pb) carbonate dating, is widely used to measure the age of carbonate minerals such as calcium, magnesium, manganese, and iron.
These minerals naturally contain uranium, which gradually transforms into lead over time.
To analyze the eggshells, researchers used a micro-laser to shave off tiny pieces of fossilized material, vaporized them, and then measured the uranium-to-lead ratio.
This ratio provided an accurate timeline for when the eggs were laid.
The breakthrough was possible after the team identified calcite, a form of calcium carbonate, within the fossilized shells, making them good candidates for U-Pb testing.
Their findings, published Thursday in Frontiers in Earth Science, mark the first reliable fossil age estimate from the Qinglongshan reserve, a site that contains over 3,000 eggs across three areas.
Many of the eggs remain partially exposed yet are still preserved in three dimensions, maintaining much of their original form.
Most of the eggs at the site are linked to the species Placoolithus tumiaolingensis of the Dendroolithidae family, classified solely from their egg structures since skeletal remains of the parent dinosaur have not been identified.
The eggs, slightly squashed spheres, measure between 120 and 170 millimeters (4.7–6.7 inches) in length, with shells no thicker than 2.4 millimeters (0.09 inch). Their porous texture may provide insights into the ancient environment as Earth’s climate cooled during the late Cretaceous.
Traditionally, paleontologists date fossils by examining surrounding sediment layers. But this approach can be unreliable, since fossils may be older or younger than the rocks encasing them.
The decision to test U-Pb dating on calcite-rich eggshells, coauthor Bi Zhao of the Hubei Institute of Geosciences explained, came unexpectedly, “Their idea to test U-Pb dating on calcite in the fossilized eggs ‘emerged somewhat serendipitously through conversations with researchers who specialize in stalagmite chronology using carbonate U-Pb methods,’” Zhao told CNN in an email. “We decided to give it a try on the Qinglongshan eggs, not expecting such clear and reliable results.”
Geochronology experts regard U-Pb analysis as one of the most accurate methods for determining deep-time ages.
“Geochronology, the science of pinpointing the ages of rocks and minerals, using U-Pb analysis ‘is by far the most precise dating technique,’” said Heriberto Rochín-Bañaga, a research associate at the University of Toronto, who has applied the method to corals and belemnites but was not involved in the new study.
According to Zhao, U-Pb dating has been successfully used on materials ranging from one million to hundreds of millions of years old.
While other decay systems exist, Rochín-Bañaga emphasized in an email that “the U-Pb system is considered to be the most accurate.”
Despite recent technological improvements, this type of dating remains uncommon for fossils. Zhao noted, “It requires highly sophisticated equipment and stringent laboratory conditions,” and even minor contamination can affect results. Moreover, while calcite-rich eggshells like these are suitable for testing, not all fossils preserve the same carbonate material. “In theory, this method could be applied to other fossils containing primary carbonate minerals. However, we have not yet attempted this,” Zhao said. “The feasibility depends on the preservation of the carbonate material and the geological context.”
The Qinglongshan reserve offers a unique look into Cretaceous ecosystems, especially dinosaur nesting habits and how species interacted with their environments. Scientists are still uncertain whether the porous quality of these eggs gave the dinosaurs an advantage, or posed a risk, as global temperatures dropped.
With this study proving that U-Pb dating works for fossilized eggs, Zhao and his colleagues plan to expand their research to other nearby Cretaceous sites.
Their goal is to better understand both the evolutionary history of these distinctive eggs and the reproductive strategies of the dinosaurs that laid them. “If widely applied,” Zhao said, “this approach could help establish a robust chronological framework for dinosaur reproduction behavior.”
