The characterization of rock strata on the basis of the fossils they contain. This involves identifying and establishing the distribution and succession of various fossil groups in order to define biozones, containing particular fossils or fossil assemblages that can generally be correlated with rock strata of a particular type in different locations. Ideally, a fossil used in biostratigraphical zoning has a limited range over geological time, so its occurrence is restricted to rock strata of a fairly narrow vertical range in the sequence. For example, the succession of numerous different ammonite species provides an important means of zoning rocks of the Mesozoic era throughout the world. Biozones thus form the basic biostratigraphy units. There are several types: for example, an assemblage zone is defined by the coincident and overlapping ranges of a particular group of fossil taxa, whereas an acme zone is defined by the exceptional abundance of one group or species.