A team of scientists have used a new dating method, known as in-situ carbonate uranium-lead (U-Pb) dating, to find out when a cluster of dinosaur eggs found in Central China were originally laid.
Traditionally, dinosaur eggs – as well as dinosaur bones and other fossil material – are dated using indirect methods. This involves dating volcanic rock, ash layers, or minerals around fossils and using these to establish a range during which they were likely deposited.
For the first time since they were discovered, U-Pb dating has allowed scientists to date the Chinese dinosaur eggs directly and assign them an accurate age of 85.91 million years (plus or minus 1.74 million years).
“We show that these dinosaur eggs were deposited roughly 85 million years ago, in the Late Cretaceous period,” said Bi Zhao, a researcher at the Hubei Institute of Geosciences and a co-author of the recent study. “We provide the first robust chronological constraints for these fossils, resolving long-standing uncertainties about their age.”
Dinosaur egg discovery
The dinosaur eggs dated in this recent study were found at a site known as Qinglongshan, located in Shiyan’s Yunyang District. More than 3,000 fossilised eggs have been found at this site, most of which are three-dimensionally preserved, largely intact, and display minimal deformation.
These eggs are thought to belong to a single species, Placoolithus tumiaolingensis. This species is what’s known as an ootaxon, meaning it’s only known from the eggs it laid, rather than its bones.
However, there’s no doubt about what kind of animal laid these eggs – based on the high porosity of their shells, scientists are confident they were laid by dinosaurs, perhaps two-legged theropod dinosaurs.
To date the cluster of 28 eggs that formed the focus of this study, Zhao’s team used U-Pb dating – a long-established dating method, but one that has only recently been adapted for use on dinosaur eggs.
“We fired a micro-laser at eggshell samples, vaporising carbonate minerals into aerosol. 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,” explained Zhao.
While only a small sample were dated, all eggs returned similar ages – ages that were also consistent with the age of the rocks surrounding the eggs.
The dating of these eggs is significant as it places them at a time of global climatic upheaval. Just prior to their deposition, in the Turonian Epoch (roughly 94 to 90 million years) temperatures dropped worldwide. This transition from warm to cooler climates is thought to have played a role in the observed trend of diminishing dinosaur diversity towards the end of the Cretaceous.
The team believes dinosaur eggs like those they’ve dated may be able to tell us more about how dinosaurs adapted during these times of dramatic change.
“Our achievement holds significant implications for research on dinosaur evolution and extinction, as well as environmental changes on Earth during the Late Cretaceous,” said Zhao. “Such findings can transform fossils into compelling narratives about Earth’s history.”
Find out more about the study: Geological Age of the Yunyang Dinosaur Eggs Revealed by in-situ Carbonate U-Pb Dating and Its Scientific Implications, published in the journal Frontiers in Earth Science.
Top image: Egg clutch sampled for chronological studies. Credit Dr. Bi Zhao
More amazing wildlife stories from around the world