How do fossils help scientists correlate rock layers from different regions?

Fossils are essential tools for correlating rock layers across different regions by utilizing the law of faunal succession. This principle observes that fossilized plants and animals appear in the rock record in a predictable, non-repetitive order over time. Because specific groups of organisms existed during distinct intervals of geologic time, finding the same or similar fossil assemblages in separated outcrops allows geologists to determine that those layers were formed contemporaneously.123

The primary method for this correlation involves the use of index fossils. An ideal index fossil is:
* Short-lived: It existed for a brief span of geologic time, providing a precise temporal marker.42
* Widespread: It had a broad geographic distribution, allowing for correlation across distant regions.42
* Distinctive: It is easily recognizable, which helps prevent misidentification.42
* Abundant: It is common enough to be reliably discovered in the field.42

By matching these fossil sequences, scientists can build a continuous record of Earth's history and assign relative ages to sedimentary strata globally.13