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
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
Floating eggs and infant stages riding on ocean currents are helpful in
giving index fossil worldwide distribution. Good index fossils need to be
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
Due to plate tectonics,
the rocks of the ocean floor are geologically young, as they are
constantly subducted and recycled into the Earth's mantle. So marine index
fossils older than about 200 million years are normally found in old
marine sedimentary strata on land.