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How does the principle of faunal succession help date fossil-bearing rocks?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

The principle of faunal succession is a fundamental concept in geochronology that allows geologists to determine the relative age of fossil-bearing rocks. It is based on the observation that assemblages of fossil plants and animals succeed one another through time in a predictable, nonrepetitive, and orderly manner.12

Because each interval of Earth's history is characterized by a unique fauna, the specific types of fossils found within a rock layer serve as a diagnostic indicator of that layer's relative age. By identifying these distinct fossil sequences, geologists can correlate strata from different geographic locations, even when the rocks are separated by great distances.231

The practical application of this principle often relies on index fossils—species that were abundant, widely distributed, and existed for only a short span of geologic time. When these fossils are found in a rock layer, they provide a reliable marker for that specific time interval. This method of dating is a cornerstone of stratigraphy and has been consistently validated by modern radiometric dating techniques.421