| Themes > Science > Paleontology / Paleozoology > Fossils And Fossilisation > Fossils and Earth History |
Toward the end of the 17th century, the naturalist Robert Hooke turned his attention to spectacular marine fossils found in his native England. Determining that these must be the remains of once-living animals, he noted that they did not resemble any living species then known, causing him to believe that life might have changed at some time in the past and that fossils might be a chronological guide to geologic history. Hooke also noted that these fossils looked more like tropical shells than species then living on British shores and wondered whether Britain's geographic latitude had also changed since the time these animals lived. The first suggestion was verified a century later, and the second three centuries later with the discovery of Continental Drift. The Earth's sedimentary strata are initially layers of muds and sands, each covering an older stratum and being covered, in turn, by a younger one (see Stratigraphy). They form, in this manner, a historical sequence, and the fossils that they contain can be arranged in time, by what has come to be known as the law of superposition. Early in the 1800's, William Smith, in England, noted that fossils were distinctive of individual beds or groups of beds in such sequences of strata, and that distinctive assemblages of fossils could be traced cross-country. Geologists soon discovered that the sequences of fossils in England could be matched with similar sequences elsewhere in the world. Any given area contains a stratigraphic record of only some part of Earth history. By combining information from many different areas, geologists can determine global Earth history. Nearly two centuries of such efforts, including the description of fossils in monographs and journals, have resulted in ever more detailed classification of the more fossiliferous part of Earth history--the last 600 million years. The smallest units of this classification, generally characterized by certain species or combinations of species, are called zones. These may be recognizable only locally, but many have been traced worldwide. Combinations of zones are the chief criteria for the recognition of worldwide units of geologic time, called stages, generally having a duration of approximately 10 million years. These in turn include the larger time periods, called systems and eras. In terms of time, the fossil record yields a relative chronology rather than an absolute one (see Geologic Time), but the development of Radiometric Age-Dating methods, using the decay of radioactive isotopes, has enabled scientists to calibrate Earth history in actual years. |
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