Fossil Record
Geologists and other scientists use fossils to correlate the ages of different rock strata (thin layers or beds of rock that differ in some way from adjacent layers) in different places on Earth. If two different rock strata contain the same set of fossil species, then the two different rock strata were probably deposited about the same time. Fossils also can give clues about the environment of Earth in the past. For example, certain fossils are only found in the ocean. When these fossils are found in a rock strata, it is a sure sign that the rock strata was deposited in an ocean even if it is now on top of a high mountain.
Fossils can also help to establish the relative ages of rocks (which rocks are older and which are younger). If a fossil species can be assigned an absolute date by radioactive dating, then that same fossil species can be used to help determine the absolute dates of other rocks that contain it. The fossil record also gives clues as to how life has evolved.
A fossil is any preserved remains of ancient life. There are several different categories of fossils. Trace fossils include such things as tracks, burrows, and coprolites (fossilized excrement). Body parts or whole bodies of organisms can be preserved by a process known as mineralization, in which minerals gradually replace the organic remains and the fossil is turned to stone.
Molds, casts, and imprints make up another category of fossils. They are formed when the sediment has solidified about an organic object and the object is subsequently dissolved, leaving a hole in the rock—a mold. Deposition of mineral matter from underground solutions may fill the hole, producing a cast. Molds of thin objects (such as leaves of ferns often found in coal) are called imprints.
Organisms with hard tissues are more likely to be preserved. Organisms that are more abundant are more likely to be preserved. Organisms that live in swamps or near water are less likely to decay when they die and are more likely to be preserved. All these factors make the fossil record somewhat incomplete. Nevertheless, the fossil record extends back at least 3.5 billion years. During this immense span of time, tens of millions of different species have lived on Earth.
The fossil record can also be used to determine the ages of rocks. The geological principle of superposition states that if rock layers are undisturbed, older rock layers are found below younger layers. If the rock layers contain fossils, then the relative ages of those fossils can be determined from the relative ages of the rock in which they were found. Then those same fossils can be used to help determine the relative ages of rocks found elsewhere. A bed of sedimentary rock can be identified by its fossils.
Using these ideas, geologists working in the first part of the nineteenth century at many different places gradually developed a theory of the history of life on Earth. This life history is now known as the geological time scale. Although the early researchers dramatically underestimated the age of Earth, they did establish the principle of determining the age of rocks by looking at the fossils found in those rocks.
Even a superficial examination of the fossil record shows that many species existed in the distant past that no longer exist today. Likewise, even an incomplete fossil record reveals that species living today did not exist in the distant past. Thus the fossil record and the geological time scale provided the background for Charles Darwin and other scientists to develop their theories of evolution.
Biological Evolution; Geological Time Scale; Morphological Evolution in Whales; Tetrapods—From Water to Land.
Bibliography
Dott, Robert H., Jr., and Roger L. Batten. Evolution of the Earth, 3rd ed. New York: McGraw-Hill, 1994.
Faul, Henry, and Carol Faul. It Began with a Stone: A History of Geology from the StoneAge to the Age of Plate Tectonics. New York: John Wiley & Sons, Inc., 1983.
Foster, Robert. Geology, 3rd ed. Columbus, OH: Charles E. Merrill, 1976.
Schneer, Cecil J. "The Rise of Historical Geology in the 17th Century." Isis. 45, no. 141 (1954):256-268.
Stanley, Stephen. Earth and Life through Time. New York: W. H. Freeman & Co., 1989.
Toulmin, Stephen, and June Goodfield. The Discovery of Time. New York: Harper & Row, 1965.
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