Dinosaur eggshells can reveal the age of other fossils

Source: Ars Technica

Overview

Like rocks, egg shells can trap isotopes, allowing us to use them to date samples.

When dinosaur fossils surface at a site, it is often not possible to tell how many millions of years ago their bones were buried. While the different strata of sedimentary rock represent periods of geologic history frozen in time, accurately dating them—or the fossils trapped within them—has frequently proven to be frustrating.

Fossilized bones and teeth have been dated with some success before, but that success is inconsistent and depends on the specimens. Both fossilization and the process of sediment turning to rock can alter the bone in ways that interfere with accuracy. While uranium‑lead dating is among the most widely used methods for dating materials, it is still an emerging technology when applied to directly dating fossils.

Dinosaur eggshells might finally have cracked a way to date surrounding rocks and fossils. Led by paleontologist Ryan Tucker of Stellenbosch University, a team of researchers has devised a method of dating eggshells that reveals how long ago they were covered in what was once sand, mud, or other sediments. That information provides the burial age of any other fossils embedded in the same layer of rock.

“If validated, this approach could greatly expand the range of continental sedimentary successions amenable to radioisotopic dating,” Tucker said in a study recently published in Nature Communications Earth & Environment.¹

This Goes Way Back

Vertebrates have been laying calcified eggs for hundreds of millions of years (although the first dinosaur eggs had soft shells — see National Geographic). What makes fossil eggshells so useful for figuring out the age of other fossils is the unique microstructure of calcium carbonate found in them (see the study in PNAS — PMCID: PMC4077312).

The way the crystals are arranged captures a record of diagenetic changes—physical and chemical alterations that occur during fossilization. These can include water damage, cracks, and fissures caused by compaction between sediment layers. Recognizing these signatures helps paleontologists determine how old the eggs (and associated fossils) are.

Sampled Sites

Dating Method

Tucker and his team applied uranium‑lead (U‑Pb) radioisotopic dating to both sets of samples. This technique can accurately date rocks ranging from 1 Myr to the age of the Earth (~4.5 Gyr) — see the overview from the University of California, Berkeley (link).

How it works

1. Eggshells and other carbonate materials trap isotopes of uranium (U) and lead (Pb).
2. Uranium isotopes are unstable and decay over time, releasing energy and losing protons/neutrons.
3. One decay pathway ends with a stable lead isotope.
4. The ratio of parent (U) to daughter (Pb) isotopes increases predictably as uranium decays.
5. By measuring these ratios and applying the known half‑lives, the absolute age of the sample can be calculated.

Take‑away

The microstructure of fossil eggshells preserves a geochemical timeline that, when paired with precise U‑Pb dating, provides robust age estimates for Cretaceous sites that were previously uncertain. This approach not only refines the chronology of the eggs themselves but also improves the broader stratigraphic framework for the surrounding sedimentary deposits.

Old Is an Understatement

The eggs from Utah are thought to have been laid by a species of oviraptor, likely Macroelongatoolithus carlylei, while the Mongolian eggs were possibly laid by a Mongolian micro‑troodontid dinosaur—small, bird‑like theropods that share ancestry with extant birds.

Trace‑element analysis showed that, despite being found on opposite sides of the planet, the structure of both eggshell samples was amazingly well preserved. Only a few micro‑fractures, caused by the pressure of overlying sedimentary layers during fossilization, were observed.

After the researchers dated both shell samples directly, they compared the results. Zircons in the sediments of the Deep Eddy site had already established its age. The nest site where the clutch of dinosaur eggs was trapped between strata of volcanic rock dated slightly older than the eggshells themselves, which were determined to be 95 million years old. As expected, the rock beneath the clutch was older, while the rock above was younger. The team noted that fractures in the eggs might have introduced minor inaccuracies.

For the Mongolian eggs, uranium‑lead dating yielded an age that was also extremely close to that of the surrounding bedrock.

An analysis of trace elements in the Teen Ulaan Chaltsai eggs revealed a surprise: a meteor likely fell to Earth around the time they were buried (≈ 99 million years ago). Even if no impact occurred at that moment, meteor dust was already present in the sediment that covered them.

“This study demonstrates that eggshell biocalcite from non‑avian dinosaurs, birds, and other egg‑laying vertebrates has the potential to serve as a reliable geochronometer in Mesozoic and Cenozoic terrestrial sedimentary basins,” — Tucker et al., Communications Earth & Environment (2025).

Next time a fossil site of indeterminate age confounds paleontologists, they need only look for dinosaur eggs.

Reference

Communications Earth & Environment, 2025. DOI: 10.1038/s43247-025-02895-w

About the Author

Elizabeth Rayne is a writer whose work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. She lives just outside New York City with her parrot, Lestat. When not writing, she enjoys shapeshifting, drawing, and cosplaying obscure characters. Follow her on Threads and Instagram: @quothravenrayne.

Footnotes

1. Tucker, R. et al. (2025). Nature Communications Earth & Environment, https://www.nature.com/articles/s43247-025-02895-w. ↩