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How do fossils help geologists determine the relative ages of sedimentary rock layers?

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Faux amphibolite — geology A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.

Fossils are essential tools for geologists to determine the relative ages of sedimentary rock layers through the law of faunal succession. This principle observes that fossilized plants and animals appear in the rock record in a predictable, nonrepetitive order over time.1

Because specific groups of organisms existed only during certain intervals of geologic time, the presence of these fossils allows geologists to correlate rock strata found in different geographic locations. The most effective fossils for this purpose are index fossils, which are characterized by:21

Short temporal range: The species existed for a relatively brief period.
Wide distribution: They are found across large geographic areas.
Distinctiveness: They are easily recognizable and abundant.23

By identifying these unique fossil assemblages, geologists can determine that rock layers containing the same fossils were deposited contemporaneously. While the principle of superposition helps establish the vertical order of layers, faunal succession provides the critical framework for matching and dating these sequences across the globe.314