A biostratigraphic unit is a unit assigned to a rock layer that contains a unique type of fossil or fossil assemblage. This unit is generally called a biological zone and can be further divided into range zones, interval zones, lineage zones, assemblage zones and enrichment zones. The fossils generally reflect the pattern of evolution over geologic time, and this evolution is not duplicated in the stratigraphic record. In this respect, the biostratigraphic unit represents a relative geologic age. In the same stratigraphic interval, several range zones, interval zones, lineage zones, assemblage zones and enrichments zones can be independently categorised based on the characteristics of the selected fossils. Biostratigraphic units are different from chronostratigraphic units and from lithostratigraphic units. Their boundaries are sometimes similar but sometimes cross each other.