Dinosaur Eggshells Infused with Uranium Offer a New Way to Date Ancient Fossil Sites
Fossils can reveal traces of an ancient world, but their exact age is not always easy to discern. Palaeontologists have traditionally relied on a range of methods to date fossil sites, some more reliable than others. But a new approach may help them unearth the past better than ever before with the help of dinosaur eggshells.
A new study published in Communications Earth & Environment has detailed two successful cases in which isotopes in fossilized dinosaur eggs enabled researchers to date fossil-bearing rocks. This process could become indispensable in regions without layers of volcanic deposits, which are commonly used to determine the age of fossils.
Read More: 70-Million-Year-Old Dinosaur Egg Reveals Ancient Nesting Clues
Finding the Age of Ancient Fossils
There are many ways to date fossil sites, but they generally fall within two categories: absolute dating (sometimes called chronometric dating) or relative dating.
Absolute dating assigns an exact age for a fossil site, often by assessing the decay of radioactive elements within rocks or minerals. For example, uranium-lead (U-Pb) dating tracks the decay of uranium isotopes into lead isotopes, and is frequently applied to the mineral zircon. Other methods, such as argon-argon dating, rely on remnants of ancient volcanism (ash beds and lava flows) to determine the age of fossil sites.
Relative dating, on the other hand, only shows whether a fossil is older or younger than other objects. It’s usually used to classify the chronological sequence of fossils at a site. One way to do this is by analyzing rock layers, where the oldest fossils are found at the bottom and younger ones are higher up.
Isotopes Within Dinosaur Eggs
Even though paleontologists have several dating methods at their disposal, not all attempts to date teeth and bone fossils end up with accurate results. Certain methods also need minerals (like zircon or apatite) that aren’t always present at fossil sites.
To get around this limitation, researchers involved with the new study turned to dinosaur eggshells as an alternative. Using U-Pb dating and elemental mapping, they measured trace amounts of uranium and lead in the calcite of the eggs, which indicated when they were buried.
“Eggshell calcite is remarkably versatile,” said author Ryan Tucker, a geologist at Stellenbosch University. “It gives us a new way to date fossil sites where volcanic layers are missing, a challenge that has limited paleontology for decades.”
The researchers tested this method on eggshells located at two Cretaceous fossil sites: the Mussentuchit Member of the Cedar Mountain Formation in Utah and the Teel Ulaan Chaltsai locality in the Eastern Gobi Basin of Mongolia.
The eggshell tests at the Utah site (which already contains ash beds) provided ages with an accuracy of about five percent relative to precise volcanic-ash dates.
Unlike the site in Utah, the Mongolian site has no datable volcanic deposits. As a result, the eggshell tests there yielded the first-ever direct age for a historic site preserving dinosaur eggs and nests, determining that the site is around 75 million years old.
No Need for Volcanic Clues
The tests in Utah and Mongolia confirm that dinosaur eggshells — not just rocks and minerals — can be used for reliable dating. With insight from elemental mapping, the researchers suggest that soon after the ancient eggs were buried in soil, they absorbed uranium from surrounding water; this uranium uptake is ultimately what allowed them to find an age for the two fossil sites.
Most importantly, dinosaur eggshells could be valuable for fossil dating in basins that lack datable volcanic evidence — including the Gobi Basin in Mongolia, Auca Mahuevo in Patagonia, and the Elliot Formation in South Africa.
“Direct dating of fossils is a paleontologist’s dream,” said co-author Lindsay Zanno, a paleontologist at North Carolina State University. “Armed with this new technique, we can unravel mysteries about dinosaur evolution that used to be insurmountable.”
Read More: How Did Dinos Lay Eggs and What Were Dinosaur Eggs Like?
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