Researchers have applied U-Pb carbonate dating directly to eggshells from the Qinglongshan site in Yunyang for the first time, determining an Upper Cretaceous age and offering a new window into ancient climate and dinosaur diversity dynamics
For the first time, scientists have dated dinosaur eggs that lay buried in rocks for millions of years, using a groundbreaking “fossil atomic clock” method.
During the Cretaceous period, the Earth experienced intense volcanic eruptions, widespread depletion of oxygen in the oceans, and several mass extinction events. Fossils from that period have been preserved and still provide scientists with valuable insights into the climates that prevailed in different parts of the world.
Now, a team of researchers in China has analyzed an extraordinary collection of such fossils: dinosaur eggs discovered at the Qinglongshan site in the Yunyang Basin in central China. For the first time, uranium-lead (U-Pb) carbonate dating has been successfully applied directly to eggshells to determine their age. Their findings were recently published in Frontiers in Earth Science.
“We have shown that these eggs fossilized about 85 million years ago, in the Upper Cretaceous,” said Dr. Bi Zhao, a researcher from the Hubei Institute of Geosciences. “We provide the first solid chronological dating for these fossils, resolving long-standing uncertainties about their age.”
New dates
Qinglongshan is the first national reserve in China dedicated to dinosaur egg fossils. Over 3,000 fossilized eggs have been documented in three excavation areas. Many of the eggs are preserved within various rock types, including breccias, mixtures of breccia and mudstone, and fine-grained flaky sandstones.
Remarkably, most of the eggs remained in their original positions and showed very little deformation. The researchers believe that most of them belong to the species Placoolithus tumiaolingensis of the Dendroolithidae family, which is known for its extremely porous eggshells. The specimen examined in the study, an egg filled with calcite, was taken from a group of 28 eggs embedded in rock rich in breccia and mudstone.
To date the egg, the team used U-Pb dating. “We used a microlaser on eggshell samples, and we identified carbon minerals in the aerosol. This was analyzed with a mass spectrometer that counts uranium and lead atoms. Since uranium decays into lead at a constant rate, we could calculate the age based on the lead that had accumulated—it’s like an ‘atomic clock for fossils,’” Zhao explained.
The results showed that the eggs in this cluster were deposited around 85 million years ago, with an uncertainty range of about 1.7 million years to either side. This means they were laid in the Late Cretaceous, a period that lasted from about 100 to 66 million years ago. These are the first fossils to be directly dated from the Qinglongshan site.
Traditionally, dating dinosaur eggs has been done using indirect methods, such as dating volcanic rocks, ash layers, or minerals around the eggs. These may have formed before or after the eggs were laid, or have undergone geological changes. The method presented here allows for precise dating of the eggs themselves, without relying on their environment. “It revolutionizes our ability to determine a global chronological age for dinosaur eggs,” Zhao said.
ancient climate
Global cooling began a few million years before the eggs were laid, during the Turonian period, about 93.9–89.8 million years ago. By the time the eggs were laid, temperatures had already dropped significantly. The shift from a warmer to a cooler climate likely caused a decline in dinosaur diversity, and may have affected the number of eggs sampled and the number of species that laid them at Qinglongshan.
“The unique pore structures of dendroolithids may represent evolutionary adaptations to this climate transition, as new egg types emerged worldwide during the cooling,” Zhao said. The pore structure of Dendroolithidae eggs, which is strikingly different from many other dinosaur eggs, may be one of those adaptations. P. tumiaolingensis may represent an evolutionary dead end where the egg-laying population was unable to successfully adapt to the cooling climate,” she added.
Although the number of eggshell samples in the study was small, all tests confirmed a similar age to the shell fragments, which is also consistent with the age of the surrounding rocks. The team will expand the sampling to include eggs from different rock layers, which could help build a regional timeline. The researchers note that dendrolithid eggs in nearby basins should be examined in the future to track dinosaur migrations.
“Our achievement has important implications for the study of dinosaur evolution and extinction, as well as environmental changes on Earth during the Late Cretaceous,” Zhao said. “Finds like this can turn fossils into fascinating evidence of Earth’s history.”
For the scientific article: "Geological age of the Yunyang dinosaur eggs revealed by in-situ carbonate U-Pb dating and its scientific implications"
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One response
How much lead and how much uranium were assumed to have been in the eggs before they were fossilized? Zero? Another number? Why? Without knowing how much lead and uranium there were in the first place, how did they calculate the age of the eggs?