Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. Because they are often rare, primate fossils are not usually good index fossils.

Because some ammonites only lived during specific periods, when geologists find such fossils, they know the age of the geological structure — this makes ammonites index fossils. Ammonites are ...

Ammonite shells are used today as index fossils, meaning they can help date other fossils that are found in the same layer of marine rock. These cephalopods make for ideal index fossils because ...