To be a useful index fossil, a species must have been rare.

This statement is false.

In order to be a useful index fossil, the fossil needs to be common so that they can be found in different sediments and therefore serve as an index. They also need to be easy to identify and be broadly distributed. If fossils don't have these characteristics, it is hard to find them and recognise them in two different sediments.

Avertical or horizontal line used in graphs is called a(n) equatorhope this