Dinosaur eggs from China found to be around 86 million years old, according to ‘atomic clock’ dating

A clutch of 28 dinosaur eggs found in the Qinglongshan fossil reserve in central China is about 86 million years old, according to scientists who used an “atomic clock” method to date the samples. Researchers said they now hope the eggs, and the technique employed to evaluate their age, might help to reveal how dinosaurs living in China’s Yunyang Basin adapted to a cooling climate.

The dating technique used on the eggs, known more formally as carbonate uranium-lead, or U-Pb, dating, is a common process for determining the age of carbonate minerals — those containing calcium, iron, manganese and magnesium. Uranium is present within these minerals, and over time, it decays into lead.

Scientists used a micro-laser to shave off bits of fossilized eggshell samples, vaporized the mineral fragments and then counted the number of uranium and lead atoms. By evaluating the ratio of uranium to lead, they were able to of determine the age of the eggs.

Recent identification of calcite — a form of calcium carbonate — in the fossil eggshells suggested that the eggs would be good candidates for U-Pb dating, researchers reported Thursday in the journal Frontiers in Earth Science. The eggs are the first fossils to be reliably dated from the Qinglongshan fossil reserve, which includes three sites containing more than 3,000 eggs, the majority of which are semi-exposed and preserved in 3D, with their original shapes largely intact.

Most of the eggs there belong to the species Placoolithus tumiaolingensis in the Dendroolithidae family, a classification that is derived from the eggs rather than from a dinosaur’s fossil skeleton. (The dinosaur that laid the eggs has not yet been identified.) The eggs are slightly flattened spheres measuring about 4.7 to 6.7 inches (120 to 170 millimeters) long, with mineralized shells that are no more than 0.09 inch (2.4 millimeters) thick.

Eggshells in this group tend to be relatively porous for dinosaur eggs, and that feature could offer clues about this ancient ecosystem during the Cretaceous period (145 million to 66 million years ago), when Earth was already starting to cool down.

Direct dating

Paleontologists often estimate the age of plant and animal fossils based on sediments where the organic material was preserved. However, fossils may arrive at a location before or after the nearby rocks, lava or ash deposits form, meaning they could be older or younger than the spot where they were found.

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,” study coauthor Bi Zhao, a researcher at the Hubei Institute of Geosciences in Wuhan, China, told CNN in an email. “We decided to give it a try on the Qinglongshan eggs, not expecting such clear and reliable results.”

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 in the University of Toronto’s department of Earth sciences. Rochín-Bañaga, who was not involved in the new research, has used the method to analyze ancient corals and belemnites, an extinct order of squidlike cephalopods.

Scientists have used U-Pb dating on rocks that are from 1 million to hundreds of millions of years old, with a high level of certainty about their results, Zhao said. There are other radioactive decay systems for ancient geologic analysis, but “the U-Pb system is considered to be the most accurate,” Rochín-Bañaga wrote in an email.

Recent advances have made U-Pb dating more accessible, but it is still not widely available for fossil analysis, according to Zhao. “It requires highly sophisticated equipment and stringent laboratory conditions,” and samples must be carefully collected and examined to avoid contamination with other material that could derail the chronology, he added. And while the presence of calcite in the eggshells made this analysis possible, it may not be the case for other types of fossils.

“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.”

A snapshot of the Cretaceous period

The Qinglongshan site is a rare terrestrial snapshot of the Cretaceous, showing dinosaur nesting behavior and how groups of Cretaceous dinosaurs interacted with their environment.

The porousness of these eggs may represent an evolutionary adaptation in this unidentified dinosaur species, but it is also unknown whether having porous eggs was a benefit or a drawback as their world cooled.

With the study establishing U-Pb dating as a viable method for determining the age of fossil eggs, the researchers plan to apply the technique to other Cretaceous sites nearby, “to better understand the origin and evolution of these distinctive eggs” as well as the nesting habits of the dinosaurs that laid them, Zhao said. With more than 200 dinosaur egg sites worldwide, of which only a handful have been accurately dated, U-Pb dating could be a valuable new tool for studying preserved examples of egg laying in dinosaurs and learning how that may have changed over time.

“If widely applied,” Zhao said, “this approach could help establish a robust chronological framework for dinosaur reproduction behavior.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine. She is the author of “Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control” (Hopkins Press).

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more.

For more CNN news and newsletters create an account at CNN.com