Many species of micro-organisms have short geologic ranges , and were widely distributed in the oceans. The microfossils serve as good index fossils or tools for worldwide time correlation. Other organisms, however, have long geologic ranges, and may have inhabited geographically restricted environments, such as reefs or tidal flats. These organisms are not useful for correlation, but they may serve as tools for interpreting ancient sedimentary environments. Short geologic range.
Many microscopic organisms are part of the floating plankton in the oceans.
These species are readily distributed over large areas by ocean currents. For example, plankton falls into any sediment that is forming on the sea floor. Distinctive and easily recognized form. Preservable, fossilizable hard parts. The fossils should be abundant enough to be collected in sufficient quantity for study.