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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.

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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.

Methods fall into one of two categories: relative or absolute.

So, researchers "normalize" the data by making a ratio with strontium-86, which is stable -- meaning it doesn't decay over time.

Dividing the isotope concentrations of all the forms of strontium and rubidium by the isotope concentration of strontium-86 generates something called the "isochron." The isochron is then plugged into a model, which uses it to turn the overall radioisotope data into a clear, linear function.

"The rate of diffusion will vary, based on the sample -- what type of rock it is, the number of cracks and amount of surface area, and so on," Hayes says.

"So, there's not a simple equation that can be applied to every circumstance.

When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.

The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.

Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.

Layers of rock build one atop another — find a fossil or artifact in one layer, and you can reasonably assume it’s older than anything above it.

And atoms of strontium-86 can diffuse more readily than atoms of strontium-87 or rubidium, simply because atoms of strontium-86 are smaller.