Researchers dated dinosaur eggs directly for the first time, placing them at 85 million years old. The findings link climate cooling to evolutionary pressures that may have doomed some species.
During the Cretaceous period, Earth experienced intense volcanic eruptions, widespread loss of oceanic oxygen, and several devastating mass extinctions. Fossils from this turbulent era still survive today, offering scientists valuable insights into the climate conditions that once shaped different regions of the planet.
A team of researchers in China recently examined one such set of fossils: dinosaur eggs uncovered at the Qinglongshan site in the Yunyang Basin of central China. For the first time, scientists have successfully determined the age of dinosaur eggs using carbonate uranium-lead (U-Pb) dating. Their findings were reported in the journal Frontiers in Earth Science.
“We show that these dinosaur eggs were deposited roughly 85 million years ago, in the Late Cretaceous period,” said corresponding author Dr. Bi Zhao, a researcher at the Hubei Institute of Geosciences. “We provide the first robust chronological constraints for these fossils, resolving long-standing uncertainties about their age.”
China’s First National Egg Fossil Reserve
Qinglongshan has been designated as China’s first national reserve dedicated to dinosaur egg fossils. More than 3,000 eggs have been discovered there across three separate sites. Many of the fossils are preserved within breccias, breccia-siltstone combinations, and fine sandstones. Most remain undisturbed in their original positions and show only minor deformation. The majority are thought to belong to Placoolithus tumiaolingensis, a member of the Dendroolithidae family, which is distinguished by eggshells with unusually porous structures. The sample used in this study was a calcite-filled egg taken from a cluster of 28 eggs set within breccia-rich siltstone.
To date the egg, the team used U-Pb dating. “We fired a micro-laser at eggshell samples, vaporizing carbonate minerals into aerosol. This is analyzed by a mass spectrometer to count uranium and lead atoms. Since uranium decays into lead at a fixed rate, we were able to calculate the age by measuring accumulated lead— it’s like an atomic clock for fossils,” Zhao explained.
Establishing a Clearer Timeline
The results showed that the eggs from this cluster were deposited around 85 million years ago, with the possibility of them having been deposited around 1.7 million years earlier or later. Their age means they’ve been laid during the Late Cretaceous, an epoch lasting from approximately 100 to 66 million years ago. They are the first reliably dated fossils from the Qinglongshan site.
Traditionally, dating dinosaur eggs involves indirect methods, such as dating volcanic rock, ash layers, or minerals around eggs. These, however, may have formed before or after the laying of the eggs, or geological processes may have altered them. The method used here allows for precise dating of eggs without having to rely on anything but the eggs themselves. “It revolutionizes our ability to establish global dinosaur egg chronologies,” Zhao said.
Cooling Climates and Evolutionary Pressure
Global cooling had started several million years before the laying of the eggs, in the Turonian epoch (lasting from approximately 93.9 to 89.8 million years ago). By the time they were laid, temperatures had declined significantly. The transition from a warm to a cooler climate was likely a factor in dinosaurs’ diminishing diversity and may have affected how many eggs were laid by how many species at Qinglongshan.
“Dendroolithids’ specialized pore structures may represent evolutionary adaptations to this climatic shift, as novel egg types emerged worldwide during cooling,” Zhao said. The pore structure of Dendroolithidae eggs, which are markedly different from many other dinosaur eggs, may be one such adaptation. “P. tumiaolingensis may represent an evolutionary dead end where the egg-laying dinosaur population failed to adapt successfully to cooling climates,” Zhao explained.
Evolutionary Dead Ends and Survival Struggles
Although few eggshell samples were examined in this study, all tests confirmed similar ages of egg fragments, which were also consistent with the age of the rocks surrounding the eggs. The team will be expanding sampling to include eggs found in different rock layers, which could help construct a regional timeline. They also said that Dendroolithid eggs in neighboring basins should be examined in the future to trace dinosaur migrations.
“Our achievement holds significant implications for research on dinosaur evolution and extinction, as well as environmental changes on Earth during the Late Cretaceous,” Zhao said. “Such findings can transform fossils into compelling narratives about Earth’s history.”
Reference: “Geological age of the Yunyang dinosaur eggs revealed by in-situ carbonate U-Pb dating and its scientific implications” by Qingmin Chen, Xing Cheng, Jian Wang, Bi Zhao, Shukang Zhang, Youfeng Ning, Gaohong Wang, Kaikai He, Wenshuo Zhang, Dongxiang Yu, Jiangli Li, Yarui Zou, Gang Chen, Min Li and Hai Cheng, 16 July 2025, Frontiers in Earth Science.
DOI: 10.3389/feart.2025.1638838
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