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Scientific fossil dating

If a fossil is found between two layers of rock whose ages are known, the fossil's age is thought to be between those two known ages.

The laser dislodged radioisotopes and other relevant isotopes, which were detected and counted.

Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.

Uranium is present in many different rocks and minerals, usually in the form of uranium-238.

Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology.

A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.

But this "age" was not only the result of a broken radioisotope system, it was contrived to agree with previously assigned dates for the samples.

The scientists analyzed the abundance of radioactive isotopes of certain elements that had leeched into the edges of buried dinosaur bone from the San Juan Basin in northern New Mexico and southern Colorado.

Sometimes only one method is possible, reducing the confidence researchers have in the results. “They’re based on ‘it’s that old because I say so,’ a popular approach by some of my older colleagues,” says Shea, laughing, “though I find I like it myself as I get more gray hair.” Kidding aside, dating a find is crucial for understanding its significance and relation to other fossils or artifacts.