Effective index fossils have several characteristics—they are widespread, distinctive, abundant, and limited in geologic time.
The law of lateral continuity states that the deposition of sediment is continuous unless there is a barrier or subsequent changes in the landscape break up layers.
This can be a challenging concept to understand, but if you imagine a basin where sediment is deposited, this principle states that the sediment will be equally deposited in this area until there is no more sediment or until the sediment reaches the edge of the basin or another barrier.
Faunal succession can be used to chronologically put a group of animals and plants together and give greater detail to the history of the Earth.
Faunal succession is also used to compare fossils of organisms in different areas on the Earth.
However, since environments change over time, this is not a perfect way of determining geologic age.