What rocks are best suited for radiometric dating

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Because lagerstätten are restricted to a narrow range of environments (where soft-bodied organisms can be preserved very quickly, e.g.by mudslides), most animals are probably not represented; further, the exceptional conditions that create lagerstätten probably do not represent normal living conditions.Relative dating (A was before B) is often assumed sufficient for studying processes of evolution, but this, too, has been difficult, because of the problems involved in matching up rocks of the same age across different continents.

Also, many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.

Fossils (Grypania) of more complex eukaryotic cells, from which all animals, plants, and fungi are built, have been found in rocks from The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B.

Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were complex, but different from any living animals.

This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.

Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only recently become available, and for only a few regions.

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