A researcher conducts sampling on a dinosaur egg fossil at the Qinglong Mountain site in Shiyan City, central China's Hubei Province, May 2024. (Hubei Institute of Geosciences in China/Handout via Xinhua)

WUHAN, Sept. 30 (Xinhua) -- Chinese scientists have determined that a dinosaur egg, unearthed in the mountains along a Yangtze River tributary in central China, is around 86 million years old using an unconventional dating method never before applied to fossil eggshells.

The method, known as carbonate uranium-lead (U-Pb) dating, involves using lasers to vaporize minerals in fossilized eggshell fragments. The resulting vapor contains uranium and lead atoms. Over millions of years, uranium gradually transforms into lead, like sand slipping through an hourglass. By measuring the proportions of uranium and newly formed lead in the minerals, scientists can determine how long the "hourglass" has been running, revealing the fossil's age.

Lead researcher Zhao Bi, from the Hubei Institute of Geosciences in China, said the method has been used to calculate the age of Earth, lunar magma and cave rocks, but using it on a fragile dinosaur egg was new.

Prior to this study, the common method was dating the rock layers in which dinosaur eggs were buried. But such an indirect method provides only an approximate time range, such as the late Cretaceous period, spanning 100 million to 66 million years ago, and lacks precision, Zhao told Xinhua.

In this new study published in the journal Frontiers in Earth Science, Zhao's team used this method to date Cretaceous dinosaur egg fossils for the first time, precisely determining that the minerals within eggshells from the Qinglong Mountain site formed between 87.65 and 84.17 million years ago.

"During this period, the global climate experienced a significant transition, gradually cooling down from the typical greenhouse conditions since approximately 93 million years ago. The findings indicate that the dinosaurs in Qinglong Mountain laid their eggs during this period of global cooling," Zhao said.

Since the 1990s, scientists have uncovered dinosaur egg fossils in Qinglong Mountain, in Shiyan City, Hubei Province. Subsequently, a national-level nature reserve and an on-site museum were established, collectively preserving more than 3,000 dinosaur egg fossils to date.

According to co-author Zhang Shukang, from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the majority of dinosaur eggs in Qinglong Mountain can be classified under one genus and species: Placoolithus tumiaolingensis.

However, Zhang noted that she continues to search for potential new egg types. Each year, she collects around 100 new eggshell samples from the site and brings them to her lab in Beijing for detailed analysis.

The dinosaur eggs in this study are flat and circular, unlike typical round, oval or oblong shapes, and have thick shells and a bi-layered structure, with the inner layer being porous.

Since eggshells are largely made of carbonate minerals, the researchers believe that the method should work on eggs from other sites and even eggs of non-dinosaur species. Similarly, other carbonate-based fossils -- or the rocks and minerals surrounding fossils -- could also be dated this way, offering new insights into evolutionary timelines, Zhao noted.

The study has attracted widespread attention within the paleontological research community, and its findings have been featured in leading journals such as Nature and Science.

Guntupalli Prasad, a palaeontologist at the University of Delhi in New Delhi, said that the direct dating method will have wide ramifications for palaeontology. "This will remove the uncertainties associated with the ages of many fossils," he was quoted as saying in a Nature report.

But Susannah Maidment, a palaeontologist at the Natural History Museum in London, said the study did not consider changes during burial and fossilization. Diagenesis, for example, can alter the original composition of materials. "Any attempt to directly date fossils must demonstrate that the mineralogy has not been altered by diagenetic processes, as this would result in an incorrect age estimation," she said.

When it comes to the limitations of this new study, Zhao admitted that it was "largely exploratory," lacking comparisons. He said the study sampled only one eggshell fossil from the lower part of a 15-meter-thick egg-bearing layer, and the researchers have not yet tested those from other layers of the same section nor from other basins near the Qinglong Mountain site.

But he believes the study is meaningful, as among the more than 200 dinosaur egg sites worldwide, only a small number have been precisely dated.

"If this method could be widely applied, it could help establish a robust chronological framework for understanding dinosaur reproductive behavior and provide insights into the ancient Earth's environmental changes, as well as the mysteries of dinosaur evolution and extinction," Zhao said. ■

This file photo taken in December 2023 shows a group of fossilized dinosaur eggs at the Qinglong Mountain site in Shiyan City, central China's Hubei Province. (Hubei Institute of Geosciences in China/Handout via Xinhua)