What is one example of an index fossil?

A trilobite could be an example of an index fossil, because they only existed for a certain period of time

trilobites and ammonites

Explanation:

Trilobites are a fossil group of extinct marine animals which became extinct around 250 million years ago.

Ammonites are a extinct group of marine invertebrates. They are molluscs in the class Cephalopoda in which we also find octopuses, squids, cuttlefishes and the nautilus.