Index fossil that is also known as indicator fossil is used to determine the age of the layers of the rocks. It says that the new layer of sedimentary rocks lie over the old layer.
The deeper we go, the older the rocks we will find. Index fossil of animals that lived or survived only for a short period of time are very useful. An example of index fossil can be Ammonites that were found in Mesozoic era that was about 25-35 million years ago.
