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Biostratigraphic dating

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Planktonic (floating) and nektonic (swimming) organisms are generally less affected by local environmental factors such as water depth, physical obstacles, or changes in substrate than are benthic species (bottom dwellers).This characteristic makes the fossils of planktonic forms—particularly calcareous nannofossils, planktonic foraminifera, dinoflagellates, and graptolites—and nektonic organisms such as conodonts excellent regional and even worldwide time markers in marine strata.Such observations led to the major divisions of the Phanerozoic time scale – the Paleozoic, Mesozoic, and Cenozoic eras – and to attempts to resolve much finer subdivisions using fossil species.These subdivisions enable time correlation – the identification of strata in different places that were deposited during the same time interval.

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The fossils are useful because sediments of the same age can look completely different because of local variations in the sedimentary environment.

In marine sediments, calcareous nannofossils are the most useful group for Jurassic (approx.

210 myrs) to recent sediments, since their preparation is cheap and the stratigraphic resolution which can be reached with them is high.

Biostratigraphers study the distribution of fossils in sedimentary strata.

They have two motives – reconstructing the history of life and developing a relative time scale for other geologic studies.