Index fossils are fossils that can be used to date rock in which they are found. For example, ammonites lived in the Mesozoic era. If ammonites are found in a rock in one part of the continent and also in a rock on the side of continent, it can be said that both rocks are Mesozoic.
Further to this, different species of ammonites lived at different times within the Mesozoic so identity of a fossil species can help narrow down when a rock was formed.
