Dinosaur egg fossil sampled for geochronology, Qinglongshan, Yunyang Basin, China. Credit: Dr Bi Zhao
A new dating method has been used by researchers to determine the age of fossilised dinosaur eggs embedded in rock at Qinglongshan in central China’s Yunyang Basin.
“We show that these dinosaur eggs were deposited roughly 85 million years ago in the Late Cretaceous period,” says Dr Bi Zhao, a researcher at the Hubei Institute of Geosciences, China.
“We provide the first robust chronological constraints for these fossils, resolving long-standing uncertainties about their age.”
Traditionally, the age of dinosaur eggs has been determined using indirect methods, such as dating the volcanic rock, ash layers, or minerals surrounding them.
Interior of Qinglongshan Dinosaur Egg Fossil Museum, China. Credit: Dr Bi Zhao
But because these materials may have formed before or after the eggs were laid and may have been altered by geological processes in the interim, this isn’t the most reliable method.
Zhao and colleagues have now shown that carbonate uranium-lead (U-Pb) dating can be used to determine the age of fossilised eggs for the first time. Their results have been published in a new study in Frontiers in Earth Science.
“We fired a micro-laser at eggshell samples, vaporising carbonate minerals into aerosol,” explains Zhao.
“This is analysed 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.”
Dinosaur egg fossil sampled for geochronology, Qinglongshan, Yunyang Basin, China. Credit: Dr Bi Zhao
It is the first reliably dated fossilised egg from Qinglongshan, where more than 3,000 semi-exposed, 3-dimensionally intact dinosaur eggs are spread across 3 sites. The 2 eggshell fragments analysed in the study were collected from one egg in a cluster of 28.
The eggs are thought to belong to Placoolithus tumiaolingensis, a dinosaur belonging to family Dendroolithidae known from its highly porous eggshells.
Fossilised egg clutch sampled for chronological studies, Qinglongshan, Yunyang Basin, China. Credit: Dr Bi Zhao
The Late Cretaceous was marked by a significant global temperature decline, which was likely a factor in the decline of dinosaur diversity seen in the fossil record.
“Dendroolithids’ specialised pore structures may represent evolutionary adaptations to this climatic shift, as novel egg types emerged worldwide during cooling,” Zhao said.
“P. tumiaolingensis may represent an evolutionary dead end where the egg-laying dinosaur population failed to adapt successfully to cooling climates.”
The team now plans to sample eggs in different rock layers to construct a timeline for the Qinglongshan egg assemblage.