Index fossils (also called type fossils or key fossils) are those that are used to define periods of geologic time. A good index fossil is one with five characteristics: ❖ it is distinctive ❖ globally widespread ❖ abundant ❖ limited to a particular geologic time and ❖ is robust and preserves well Most fossil-bearing rocks formed in the ocean as being buried in sediment is easiest there. So the major index fossils are marine organisms (global and universal) and only a very few terrestrial organisms are index fossils (limited to young rocks in specific regions and not universal for example: quickly evolving rodent teeth). Floating eggs and infant stages riding on ocean currents are helpful in giving index fossil worldwide distribution. Good index fossils need to be both --abundant and vulnerable Abundant and globally widespread over a relatively short period of time. Vulnerable to environmental change and extinction. This combined "boom-and-bust character" is what makes for a good index fossil. Some index fossils are small or microscopic, part of the floating plankton in the world ocean. Their tiny bodies rained down all over the ocean, they can be found in all kinds of rocks. The petroleum industry has made great use of index microfossils, and geologic time is broken down in quite fine detail by various schemes based on calcareous nannofossils, conodonts, diatoms, foraminifera and radiolarians