How does biostratigraphy use fossils to divide geologic time?

Biostratigraphy is the branch of paleontology and stratigraphy that uses the identification of fossils within sedimentary rock strata to determine their relative geologic age.12

This method relies on the law of faunal succession, which observes that fossilized plants and animals appear and succeed one another in a predictable, nonrepetitive order throughout Earth's history. Because specific groups of organisms existed only during particular intervals, their presence in a rock layer acts as a diagnostic marker for that time period.34

To divide geologic time, geologists use:

• Index Fossils: These are the primary tools for correlation. A useful index fossil is abundant, widely distributed, easily recognizable, and existed for only a short span of geologic time.53
• Biostratigraphic Boundaries: These are defined by the specific points where certain fossil types appear or disappear in the rock record. These boundaries serve as the basis for defining units of the geologic time scale, such as eons, eras, periods, epochs, and ages.65

By identifying these fossil assemblages, scientists can correlate rock strata from different geographic locations, effectively piecing together a global history of Earth.34