For the first time, scientists have directly dated a fossilized dinosaur egg using in situ uranium–lead (U–Pb) isotopic analysis – a method that could improve the precision of fossil dating and reduce reliance on surrounding rock layers. The result places the egg, found in a nesting area in Shiyan, China, at 85.9 ± 1.6 million years old, corresponding to the middle of the Late Cretaceous period.
The study, published in Frontiers in Earth Science, highlights the potential of direct fossil dating using in situ isotopic analysis. Traditional approaches estimate the age of fossils by dating nearby volcanic ash or sedimentary layers – a process that can misrepresent the actual time of fossilization. In contrast, the new method targets the fossil itself.
The team applied laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) to micrometer-scale regions of the eggshell, which had accumulated uranium-bearing secondary carbonate minerals during fossilization. By analyzing the decay of uranium isotopes into lead within these mineral coatings, the researchers established a direct chronological framework for the egg.
“We show that these dinosaur eggs were deposited roughly 85 million years ago, in the Late Cretaceous period… resolving long-standing uncertainties about their age,” said corresponding author Bi Zhao, a researcher at the Hubei Institute of Geosciences, in a press release.
To assess the reliability of the fossil’s age, the researchers combined isotopic dating with microstructural imaging techniques. Scanning electron microscopy and cathodoluminescence revealed that the eggshell calcite was well preserved, with no signs of significant recrystallization or deformation. This preservation supports the assumption that the uranium-bearing minerals analyzed by LA-ICP-MS reflect the original fossilization timeline, rather than later geological alterations.
The method has previously been used for dating fossilized bones, teeth, and geological materials, but not for dinosaur eggshells. The researchers focused on well-preserved calcite fragments with minimal deformation, indicating low diagenetic alteration – an important factor, as secondary mineralization can complicate radiometric dating.
The authors acknowledge the concern but cite previous findings of limited regional volcanic activity and minimal signs of alteration in the sampled material. Still, they plan further studies to validate the approach across additional fossil sites and specimens.
If verified more broadly, the technique could offer a powerful new tool for palaeontology, eliminating long-standing uncertainties associated with indirect dating methods. “This will remove the uncertainties associated with the ages of many fossils,” commented Guntupalli Prasad, a palaeontologist at the University of Delhi.
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