between the appearance of species chosen at the base of the zone and the appearance of other species chosen at the base of the next succeeding zone. Oppel's zones are named after a particular distinctive fossil species, called an index fossil. Index fossils are one of the species from the assemblage of species that characterize the zone.

Biostratigraphy uses zones for the most fundamental unit of measurement. The thickness and range of these zones can be a few meters, up to hundreds of meters. They can also range from local to worldwide,