What makes a fossil an index fossil ?

Explanation:

A fossil is an index fossil when it is distinctive, abundant, and limited in geological areas. A good index fossil lived for a short period of time so that the range of ages of the strata in which the index fossil is located is small. A good index fossil is found in a wide geological range.

A fossil is an index fossil when animals or plants preserved in the rock record of the Earth is characteristic by a particular span of geologic time or environment.