Which statement explains why ammonites are a good example of an index fossil?

(1 point)
O Ammonites existed only in the oceans.
O Ammonites existed in a small geographic area.
O Ammonites existed for a relatively limited period of time.
O Ammonites existed long before the dinosaurs.

The statement "Ammonites existed for a relatively limited period of time" explains why ammonites are a good example of an index fossil.