Index fossils are fossils that are found often in many different areas of the world. These types of fossils help to date other fossils that have been found in the same sedimentary layer of the Earth.
Index fossils are also known as indicator or zone fossils since scientists and archaeologists use them to identify geological periods and stages. They work on the notion that if one type of species lived on Earth during one time period, and if a fossil of said species is found in many different areas, it must be from the same geological area. This allows archaeologists to determine the age of fossils found near the index fossils.