Interesting facts about index fossils

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.

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.