Interesting facts about index fossils

A good index fossil is one with four characteristics: it is distinctive, widespread, abundant and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are marine organisms, but certain land organisms are useful in young rocks and in specific regions.

Index fossils are very important to relative dating, as they serve as great time markers because they have a short half-life and are widespread around the world.