How do you date a dinosaur egg? [You just ask it out for dinner – Ed.] So far, paleontologists have used indirect dating methods to find out how old dinosaur eggs are. Surrounding rock, volcanic ash and biostratigraphy (comparing other fossils in the same soil/rock layer) have helped us age egg fossils. This is because fossilised eggs usually lack enough radioactive isotopes to date them directly.
Now, in China, paleontologists have successfully directly dated dinosaur egg fossils themselves. Scientists used carbonate uranium-lead (U–Pb) dating, rather than relying on the surrounding rocks. The method revealed that the eggs being studied were about 85 million years old.
The Method That Changed Egg Dating
Indirect dating may not be entirely accurate, as those layers might not match the exact time the eggs were laid. The team, led by Dr. Bi Zhao and colleagues, were able to directly examine the eggshell fragments. “Advances in carbonate U-Pb absolute dating have enabled in-situ dating of dinosaur bones“, they wrote in a new study published last month in Frontiers.
They fired a micro-laser into the calcite in the shell and vaporized it. They then measured the ratio of uranium to lead in the resulting gas. And because uranium decays to lead at a known rate, that gives a “clock” inside the egg.
What have we found about the egg itself
The site holds over 3,000 partially exposed eggs across multiple beds, making it one of the richest dinosaur-egg reservoirs in China. Because the Earth’s climate was cooling before the time the eggs were laid, researchers suggest that cooling might have driven the evolution of more specialised eggshell pore structures, potentially to deal with different moisture or gas exchange needs.
Photos of dinosaur eggshell under the microscope. The eggshell unit is slender and symmetrical (rarely asymmetrical). It is divided into two branches in the middle part of the eggshell. The compact layer accounts for 1/10–1/4 of the eggshell. Image Credit: Qingmin Chen et al
The dating result also helps close the gap in the regional stratigraphy. The age obtained matches (within error margins) what indirect dating of surrounding layers had suggested, giving new confidence in previous age estimates.
Studying dinosaur eggs can reveal much about the past
Knowing the precise age helps scientists tie those eggs to specific climate and ecological contexts, such as global cooling trends, volcanic events, or shifts in dinosaur ecology. The idea that shell pores evolved in response to changing climate suggests that dinosaur reproductive strategies were more sensitive and responsive than sometimes assumed.
“Dinosaur eggs, abundant in Upper Cretaceous terrestrial strata, provide critical insights into paleoenvironments, climate, and biotic evolution.”
“The results aim to establish a robust chronological framework for the Qinglongshan egg assemblage for the first time, enhancing understanding of Late Cretaceous terrestrial ecosystems in China’s interior and their response to global environmental changes“, the researchers wrote. “This study underscores the potential of dinosaur egg fossils as proxies for reconstructing the “Cretaceous World” in terrestrial settings.“