Biostratigraphic units (biozones) are bodies of rock strata that are defined or characterized on the basis of their contained fossils.

Biostratigraphic units exist only where the particular diagnostic biostratigraphic feature or attribute on which they are based has been identified. Biostratigraphic units are, therefore, descriptive units based on the identification of fossil taxa. Their recognition depends on the identification of either their defining or characterizing attributes. Biostratigraphic units may be enlarged to include more of the stratigraphic record, both vertically and geographically, when additional data are obtained. In addition, since they depend on taxonomic practice, changes in their taxonomic base may enlarge or reduce the body of rocks included in a particular biostratigraphic unit. Biostratigraphic units are geographically as extensive as their particular diagnostic taxa.

A biostratigraphic unit may be based on a single taxon, on combinations of taxa, on relative abundances, on specified morphological features, or on variations in any of the many other features related to the content and distribution of fossils in strata. The same interval of strata may be zoned differently depending on the diagnostic criteria or fossil group chosen. There are thus several kinds of biostratigraphic units.

Because of this diversity of possible biostratigraphic units, gaps or overlaps frequently occur both vertically and laterally between the different kinds of biozones, between biozones based on different fossil groups, or even between biozones of the same kind or based on the same fossil group.

Biostratigraphic units are distinct from other kinds of stratigraphic units in that the