SCIENTIFIC AMERICAN — On the Atlantic coast of the U.S., archaeologists found oyster shells left by Native Americans more than 4,000 years ago. In Morocco, paleontologists excavated the fossils of a dinosaur that roamed Earth 168 million years ago. How did the researchers determine these ages? When examining remnants from the past, experts use radiometric dating, a versatile technique that involves counting radioactive atoms of certain elements that are still present in a sample. The particular elements studied, as well as the details of the process, depend on the approximate age of the object that scientists hope to date.
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Originating in Earth’s mantle, some radioactive elements reach the surface through volcanic processes and become trapped inside mineral crystals in soil and rock. Over the course of millions of years, uranium 235 and uranium 238, for example, undergo multistep decays to isotopes of lead, making them ideal for paleontology: researchers can determine the age of a sample by measuring the ratio of lead to uranium isotopes. But using this technique to date fossils from creatures that lived millions of years ago, such as dinosaurs, is far from straightforward. “Fossils themselves usually can’t be dated directly,” says Sarah Gibson, a paleontologist at St. Cloud State University, who studies fish evolution about 230 million to 150 million years ago, during the early Mesozoic era.
Read more: How Do Scientists Determine the Ages of Human Ancestors, Fossilized Dinosaurs and Other Organisms?
