How does the principle of faunal succession help arrange fossils in time order?

The principle of faunal succession states that fossilized plants and animals appear in the rock record in a predictable, nonrepetitive, and orderly sequence over time. This principle allows geologists to arrange fossils in chronological order and correlate rock strata across different geographic locations.12

The process functions through the following mechanisms:

• Relative Dating: Because specific faunas are unique to particular intervals of geologic time, the presence of certain fossils indicates the relative age of the rock layer in which they are found.2
• Validation: As a secondary principle, faunal succession relies on other sequence-determining methods, such as the law of superposition, to establish the initial relative order of sedimentary layers. Once this order is established, the consistent appearance of specific fossil assemblages confirms the temporal sequence.2
• Correlation: Geologists use index fossils—species that were geographically widespread but existed for only a short period—to match and date separated rock outcrops that share the same fossil content.2

This method provides the fundamental basis for the geologic time scale, which has been consistently validated by modern radiometric dating.12