- Principle of faunal succession
The principle of faunal succession is based on the observation that sedimentary rock strata contain
fossil ised 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 fossilisedNeanderthal bone will never be found in the same stratum as a fossilisedMegalosaurus , 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. [As recounted in Simon Winchester, "The Map that Changed the World" (New York: HarperCollins, 2001), pp. 59-91.] The fossil content of rocks together with thelaw 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 thefossil record by more modern versions. For instance, paleontologists investigating the evolution ofbird s predicted thatfeather s would first be seen in primitive forms on flightless predecessor organisms such asfeathered 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. [http://www.nature.com/nature/journal/v420/n6913/fig_tab/nature01196_F5.html Mingke Yu, et. alia,"The morphogenesis of feathers", "Nature" 420, (21 November 2002), pp. 308-312] .]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 thefossil s contained within them. InCenozoic strata, fossilized tests offoraminifera 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 andostracod a was compiled by M. B. Hart (1972).References
ee also
*
Law of superposition
*Principle of original horizontality
*Principle of lateral continuity
*Principle of cross-cutting relationships
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