Fossils can reveal traces of the ancient world, but their exact age is not always easy to determine. Paleontologists have traditionally relied on a number of methods for dating fossils, some more reliable than others. But a new approach could help them unearth the past better than ever before, using dinosaur eggshells.
New research published in Connection Earth and environment detailed two successful cases in which isotopes in fossilized dinosaur eggs allowed researchers to date rocks containing the fossils. This process could be invaluable in regions where there are no layers of volcanic sediment that are typically used to determine the age of fossils.
Read more: 70-million-year-old dinosaur egg reveals ancient nesting clues
Determining the age of ancient fossils
There are many ways to date fossils, but they generally fall into two categories: absolute dating (sometimes called chronometric dating) and relative dating.
Absolute dating determines the exact age of a fossil, often by assessing the decay of radioactive elements in rocks or minerals. For example, uranium-lead (U-Pb) dating tracks the decay of uranium isotopes into lead isotopes and is often applied to the mineral zircon. Other methods, such as argon-argon dating, rely on residues ancient volcanism (ash beds and lava flows) to determine the age of fossil sites.
Relative dating, on the other hand, only shows whether fossil older or younger than other objects. It is commonly used to classify the chronological sequence of fossils at a particular site. One way to do this is to analyze rock layers, with the oldest fossils at the bottom and younger ones at the top.
Isotopes in dinosaur eggs
Although paleontologists have several dating methods at their disposal, not all attempts to date tooth and bone fossils result in accurate results. Some methods also require minerals (such as zircon or apatite) that are not always present in fossil sites.
To get around this limitation, the researchers involved in 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 eggs' calcite, indicating when they were buried.
“Eggshell calcite is surprisingly versatile,” said author Ryan Tucker, a geologist at Stellenbosch University. “This gives us a new way to date fossil sites where volcanic layers are missing, a problem that has limited paleontology for decades.”
The researchers tested this method on eggshells found in two Cretaceous fossils: the Mussentuchit Member of the Cedar Mountain Formation in Utah and the Til Ulaan Chaltsai locality in the eastern Gobi Basin of Mongolia.
Eggshell tests at a site in Utah (where there are already ash layers) determined the age with an accuracy of about five percent relative to the exact dates of volcanic ash formation.
Unlike the Utah site, the Mongolian site does not have dateable volcanic deposits. As a result, analyzes of eggshells made it possible for the first time in history to determine the direct age of a historical site where dinosaur eggs and nests were preserved, and determine that the site is about 75 million years old.
No need for volcanic tips
Tests in Utah and Mongolia confirm dinosaur eggshells are more than just stones and minerals – can be used for reliable dating. Based on elemental mapping, the researchers suggest that soon after the ancient eggs were buried in the soil, they absorbed uranium from the surrounding water; it was this uranium uptake that ultimately allowed them to date the two fossil sites.
Most importantly, dinosaur eggshells may be valuable for dating fossils in basins that lack dateable volcanic evidence, including the Gobi Basin in Mongolia, Auca Mauevo in Patagonia, and the Elliot Formation in South Africa.
“Direct dating of fossils is a paleontologist's dream,” said co-author Lindsey Zanno, a paleontologist at North Carolina State University. “Armed with this new technology, we can unravel mysteries of dinosaur evolution that were previously intractable.”
Read more: How did dinosaurs lay eggs and what were dinosaur eggs like?
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