What are index fossils primarily used for?

Index fossils are primarily used to establish and compare the relative ages of rock layers. These fossils are from organisms that lived during specific, well-defined time periods and were widespread geographically. Because of their distinct characteristics and the limited time frames in which they existed, paleontologists and geologists can use index fossils to identify the relative ages of the rock layers in which they are found.

When different rock layers contain the same index fossils, it indicates that those layers were deposited around the same time, allowing scientists to correlate rock formations across different locations. This process aids in piecing together the geological history of an area and understanding how different layers relate to one another chronologically. The use of index fossils is essential in creating the geologic time scale and understanding the history of life on Earth in a relative sense, rather than determining specific ages, which is the focus of absolute dating methods.