In the Cretaceous period, Earth was plagued by widespread volcanic activity, oceanic oxygen depletion events, and mass extinctions. Fossils from that era remain and continue to give scientists clues as to what the climate may have looked like in different regions.

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 were 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.