How can fossils help scientists compare the ages of rock layers in different places?

Fossils allow scientists to compare the ages of rock layers in different locations through the principle of faunal succession. This observation notes that fossilized plants and animals appear in the rock record in a predictable, orderly sequence over time. Because specific groups of organisms existed only during certain intervals, finding the same fossils in separated rock outcrops indicates that those layers were likely formed during the same period.12

To facilitate this, geologists rely on index fossils, which are particularly useful for correlation. An ideal index fossil is:32
* Distinctive: Easily recognizable to avoid misidentification.32
* Abundant: Sufficiently common to be found easily.32
* Widespread: Distributed across large geographic areas.32
* Short-lived: Existed for a brief span of geologic time, ensuring that its presence pinpoints a specific interval.32

By identifying these unique fossil assemblages, scientists can correlate strata across different regions, effectively piecing together a global history of Earth’s geologic time.21