Principle of faunal succession

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The principle of faunal succession is based on the observation that sedimentary rock strata contain fossilised flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances. A fossilised Neanderthal bone will never be found in the same stratum as a fossilised Megalosaurus, for example, because the two species lived during different geological periods, separated by many millions of years. This allows for strata to be identified and dated by the fossils found within.

This principle, first identified in the early 1790s by the geologist William Smith, is of great importance in determining the relative age of rocks and strata.^[1] The fossil content of rocks together with the law of superposition helps to determine the time sequence in which sedimentary rocks were laid down.

The theory of evolution powerfully explains the causal mechanism of the observed faunal and floral succession preserved in rocks. Archaic biological features and organisms are succeeded in the fossil record by more modern versions. For instance, paleontologists investigating the evolution of birds predicted that feathers would first be seen in primitive forms on flightless predecessor organisms such as feathered dinosaurs. This is precisely what has been discovered in the fossil record: simple feathers, incapable of supporting flight, are succeeded by increasingly large and complex feathers.^[2]

In practice, the most useful diagnostic species are those with the fastest rate of species turnover and the widest distribution; their study is termed biostratigraphy, the science of dating rocks by using the fossils contained within them. In Cenozoic strata, fossilized tests of foraminifera are often used to determine faunal succession on a refined scale, each biostratigraphic unit (biozone) being a geological stratum that is defined on the basis of its characteristic fossil taxa. An outline microfaunal zonal scheme based on both foraminifera and ostracoda was compiled by M. B. Hart (1972).