The diagram below is set up to help us understand how index fossils can be used. In the diagram there are two locations where the same rock strata have been found. In Layer C, a fossil is found from a horn coral that lived between 261-325 million years ago. This could give us a very large range of time for the relative age of this rock. However, the same rock formation is found in another location with a trilobite that lived between 251-285 million years ago. Because they formed in the same rock we can make the assumption that their species lived at the same time period. We can, therefore, narrow our age estimate of the rock formation by including only the time frame that overlaps from both species. In this case, that time frame is 261-265 million year ago.