==== Age v Zones ====

Biozones are the primary tool of correlation because they describe what is seen in the rock record.

Ages are an interpretation based on the known (at the time) chronological position of the biozone. As has been shown above, the positions of chronostratigraphic boundaries can and often do change.

It is perhaps an understandable mistake to make, but geoscientists should really avoid attempting to correlate ages between sections and instead try to rely more on the biozones or individual bioevents. By dealing with biozones alone, confusions which have arisen recently (like the Campanian-Maastrichtian boundary issue described above) may be avoided.

**Chapter Summary**

Chronostratigraphic units (“ages”) are interpretations (which can and do change) and should be avoided as correlation tools. Biozones are the primary tools of correlation and can be defined using several parameters within a fossil(s) stratigraphic range, including the total range of the taxon, successive first or last appearances of two or more taxa and the overlap of individual taxa ranges. Well-defined, “standard” biozones based on planktonic/nektonic organisms should be the primary tools used for correlation at global and (if possible) local scales. Latitudinal control over biozones should be noted. Local biozones and high-resolution biozones can be based on more “pragmatic” fossil occurrences and “ecostratigraphy” with less wide distribution patterns, but are valuable at the sub-basin or field level.