A zone could consist of the total time of existence of a fossil, it could consist of the time where two or more fossils coexist, it could be defined as the time between the origination of one fossil a...A zone could consist of the total time of existence of a fossil, it could consist of the time where two or more fossils coexist, it could be defined as the time between the origination of one fossil and the extinction of a different fossil, etc.
If we can track the evolution from species to species then we can easily pinpoint the beginning and end of one species and the transition to a new one . This combination allows us to identify an index...If we can track the evolution from species to species then we can easily pinpoint the beginning and end of one species and the transition to a new one . This combination allows us to identify an index fossil in the strata and have a more precise idea of what window of time we are looking at for the age of the rock unit.
