Which is a trait of a index fossil ?

Found in a limited time span is the trait of an index fossils.

Explanation:

Index fossils are also called as guide fossils or indicator fossils. It is the most common fossils that have a limited time of period. They help to hide other, if a fossil from an unknown time is near a known time fossil, it can be considered that both species were formed around the same time.

In other words, it is a ancient remains of a plant or animal that lived during a specified geological time and that can be used to know the existing rock formations.