New ‘atomic clock’ method used to date 85-million-year-old dinosaur eggs

By Stephen Beech

Dinosaur eggs laid 85 million years ago have been accurately dated using a new "atomic clock for fossils" method.

The eggs could improve understanding of Earth's climate when dinosaurs roamed the planet, say scientists.

They explained that in the Cretaceous period, when the eggs were laid, Earth was plagued by widespread volcanic activity, oceanic oxygen depletion events, and mass extinctions.

Fossils from that era continue to give scientists clues as to what the climate may have looked like in different regions.

Chinese researchers examined dinosaur eggs found at the Qinglongshan site in the Yunyang Basin, central China.

It is the first time that dinosaur eggs have been dated using carbonate uranium-lead (U-Pb) dating.

Corresponding author Dr. Bi Zhao, a researcher at the Hubei Institute of Geosciences, said: “We show that these dinosaur eggs were deposited roughly 85 million years ago, in the Late Cretaceous period.

“We provide the first robust chronological constraints for these fossils, resolving long-standing uncertainties about their age.”

Qinglongshan is China’s first national dinosaur egg fossil reserve, with more than 3,000 fossilized eggs spread across three sites.

Most fossils are embedded in different stones and the eggs have mostly remained in their original location, showing only minimal deformation.

The majority of the eggs are thought to belong to a single species, Placoolithus tumiaolingensis, which belongs to the family Dendroolithidae, a group characterized by highly porous eggshells.

The sampled calcite-filled dinosaur egg fossil came from a cluster of 28 eggs embedded within breccia-bearing siltstone.

To date the egg, the team used U-Pb dating.

Dr. Zhao said: “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."

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The results, published in the journal Frontiers in Earth Science, showed that eggs from the 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 around 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.

But they may have formed before or after the laying of the eggs, or geological processes may have altered them.

Scientists say the new method allows for "precise" dating of eggs without having to rely on anything but the eggs themselves.

Dr. Zhao said: “It revolutionizes our ability to establish global dinosaur egg chronologies."

Global cooling had started several million years before the laying of the eggs, in the Turonian epoch - lasting from around 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 species at Qinglongshan, according to the research team.

Dr. Zhao said: “Dendroolithids’ specialized pore structures may represent evolutionary adaptations to this climatic shift, as novel egg types emerged worldwide during cooling.

"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."

Although few eggshell samples were examined in the study, all tests confirmed similar ages of egg fragments, which were also consistent with the age of the rocks surrounding the eggs.

The team plans to expand their sample to include eggs found in different rock layers.

Dr. Zhao added: “Our achievement holds significant implications for research on dinosaur evolution and extinction, as well as environmental changes on Earth during the Late Cretaceous.

“Such findings can transform fossils into compelling narratives about Earth’s history.”