Fossil Dinosaur Eggshells Unlock a New Clock for Dating Prehistoric Life

Fossil dinosaur eggshells unlock a new clock for dating prehistoric life

Scientists report that fossil eggshells can absorb radioactive elements and be dated directly, offering a powerful new tool to place ancient ecosystems on a more accurate timeline. Published in Communications Earth & Environment, the study shows that uranium incorporated into eggshell minerals can be used with uranium-lead (U-Pb) methods to estimate when eggs were buried and, by extension, when the animals lived.

Why this matters

Until now, most fossil ages came from dating volcanic ash or igneous layers associated with fossil-bearing sediments. Uranium decays to lead at a known rate, so geologists use the uranium-to-lead ratio to determine rock ages. But many important fossil sites lack volcanic material, leaving paleontologists to rely on indirect estimates. Directly dating fossil material would reduce uncertainty and sharpen timelines for evolutionary and ecological questions.

What the team did

Past work showed that modern and relatively recent eggshells absorb uranium from surrounding sediment after burial. The new study tested whether dinosaur eggshells, tens of millions of years old, preserve the same signature. Researchers analyzed two sets of eggshells.

The first set, from Utah, belonged to parrot-like oviraptorosaurs and was buried between two volcanic ash beds previously dated to about 99 million years. U-Pb dating of the eggshells returned ages near 97 million years. The modest difference was attributed to eggshell porosity and details of burial and preservation, but the close agreement validates the approach where ash beds are absent.

Lindsay Zanno, a coauthor and paleontologist at the North Carolina Museum of Natural Sciences, notes that reconstructing life history depends on accurate timelines. She says the findings show eggshell dating can be a reliable complement to traditional methods.

Testing uncertain sites

Encouraged by the Utah result, the team dated eggshells from Teel Ulaan Chaltsai in Mongolia's Gobi Desert. That site had been variously assigned an age anywhere from the Early to Late Cretaceous (roughly 145 to 66 million years ago). Eggshell U-Pb dates cluster around 75 million years, placing the nesting activity in the Late Cretaceous and resolving a long-standing uncertainty about the site.

University of Calgary paleontologist Darla Zelenitsky, who was not involved in the study, cautions that well-preserved dinosaur eggshells are relatively rare, but says that when present, they could be crucial for dating egg-rich sites that lack volcanic layers.

Implications and next steps

The method could be applied to many egg-bearing formations worldwide, including South Africa's Elliot Formation and Patagonia's Auca Mahuevo nesting sites, where abundant eggs exist but volcanic dating markers are scarce. The authors emphasize that more work is needed to refine how eggshells incorporate radioactive elements and to develop checks that confirm a measured age is robust.

Overall, eggshell U-Pb dating offers a promising new route to tighten the timeline of dinosaur evolution and paleoecology. The research was supported in part by the National Geographic Society, which funds Explorers conducting field and laboratory work.

Key takeaways

• Fossil eggshells can trap uranium and be dated with U-Pb methods.
• Testing produced ages close to independently dated ash beds (Utah: ~97 vs ~99 million years).
• Application to a Mongolian site placed nesting activity at about 75 million years ago.
• The technique could resolve ages at other egg-rich sites that lack volcanic layers, improving evolutionary timelines.