Nannofossils commonly occur in astronomically large numbers in fine-grained marine deposits. This fact, coupled with the rapid evolution that characterizes the group and the succession of distinctive assemblages, makes them extremely useful for dating Mesozoic and Cenozoic marine strata.

Cenozoic nannofossils are best known; consequently zonation of this interval is the most satisfactory. Two recent compilations of Cenozoic nannofossil zones list 46 and 49 zones, respectively, for a time period of 65 million years. Most zones can be recognized in marine sediments of the different continents and ocean basins. Local and regional zonations are possible on a finer scale in many areas, because of the many "provincial" species present in hemipelagic sediments on several continents.

Mesozoic nannofossils are less well known, although considerable work has been done on the diverse assemblages from Upper Cretaceous chalks and marls from which several hundred species have been described. Lower Cretaceous and Jurassic deposits contain appreciably less-diverse nannofossil assemblages. Several partial zonations have been proposed for the Jurassic and Cretaceous, but these are considered only preliminary and are not entirely satisfactory.

The immediate need in further nannofossil studies is for systematic studies in the Mesozoic for biostratigraphic purposes. In the Cenozoic the most profitable areas of research probably are to determine the environmental significance of the many "provincial" and hemipelagic species. In addition, much useful information can be derived from intensive studies on individual lineages such as have been undertaken with the genera Chiasmolithus, Helicopontosphaera, and Sphenolithus.

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