First an INDEX FOSSIL is any animal or plant preserved in the rock, record of the Earth that is characteristic of a certain geologic time or environment.
Guiding us with the definition above some phrases can be:
- The organisms are distinctive
- widespread
- abundant
- limited in geologic time
- extinct
- had hard parts
- generally large
For example, Trilobites are a very good index fossil, because they lived in all parts of the ocean, had a hard and large shell, and were enough species to compare in the time range they were alive.