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Fossils are remains of prehistoric organisms. Preserved by burial under countless layers of sedimentary material, they are a record of the history of life beginning approximately 3.5 billion years ago, the study of which is called paleontology.
Some fossils are abundant in the strata of the Earth's crust. The chalk cliffs of Dover, England, and the Niobrara Chalk of Kansas are composed of complex platelets of algae, so small that millions fill a cubic millimeter. Shells of invertebrate marine animals, such as brachiopods, bryozoans, clams, snails, corals, and echinoderms, are preserved in many beds of limestone, and the bones and teeth of vertebrates are sometimes so numerous that they form deposits called "bone beds." In other sedimentary rocks, such as the majority of the world's red beds, shells and bones are rarely found, although tracks and burrows may be abundant. Fossils are uncommon in sedimentary rocks of Precambrian time, although some rocks of the latter part of that era have yielded a moderately diverse assemblage known as the Ediacaran fauna.
Kinds of Fossils
Entire or partial bodies of organisms are called body fossils. In contrast, marks left in rock by the activities of organisms are called trace fossils. These include artifacts, burrows, feces, tracks, and trails. Microfossils are studied in the field of micropaleontology. A related study of microscopic spores, pollen grains, and cysts extracted from sediment by hydrofluoric-acid treatment is called palynology. Chemical paleontologists study the record of organic macromolecules, the presence of which in rocks may reveal the existence of certain groups of organisms in the distant past.
How and Where Fossils Form
The Earth is teeming with life, all of it searching for food. As a result, very little digestible organic matter escapes destruction, and indigestible skeletal material, such as shells, bones, and teeth, has a much better chance of burial and preservation. Shell material is typically composed of calcium carbonate, as are mollusk shells; teeth and bones are composed of calcium phosphate. The highly indigestible organic jackets of spores and pollen grains also commonly escape destruction. Such materials form most of the body fossils common in layered rocks.
Much rarer are accumulations of sediment in settings from which scavengers are excluded and in which the bodies of plants and animals, carried in from outside, may retain their general form. These are of greatest value to the paleontologist because they give the most comprehensive view of past life. Certain rock formations are well known for this reason. One is the Precambrian Gunflint Chert on the north shore of Lake Superior. It contains well-preserved bacteria and blue-green algae approximately 2 billion years old. Another, the Burgess Shale of British Columbia, contains carbonaceous films of soft-bodied marine worms, crustaceans, and more unusual life forms of the Cambrian Period. At the Carboniferous Mazon Creek locality in Illinois, both land plants and marine invertebrates are preserved. The Holzmaden oil shale of southern Germany, of Early Jurassic Period age, is well known for its many fish and crinoids, as well as for the Ichthyosaurus and other marine reptiles that have been found there. Even better known is the Late Jurassic Solnhofen limestone of southern Germany, where the quarries, worked for lithographic stone, have yielded not only large numbers of well-preserved jellyfish and horseshoe crabs, but also flying reptiles (pterosaurs) and one of the earliest birds, Archaeopteryx.
Some of the most spectacular fossils of the Cretaceous Period, including fish, marine reptiles, pterodactyls, and birds, have come from the Smoky Hill Chalk, near Hays, Kans. A remarkable Eocene lake fauna has been uncovered in the Green River Formation, near the town of Fossil, Wyo. Along the shore of the Baltic Sea, insects, spiders, and the like are found preserved in amber, fossilized resin exuded from trees.
The most information on Pleistocene Epoch faunas, from vultures to bison, wolves to saber-toothed cats to beetles, has come from tar seeps, such as the La Brea Tar Pit in Los Angeles, where prehistoric animals got mired and embalmed in the tar, as they do in modern tar pits. Most extraordinary are the few finds, in Alaska and Siberia, of prehistoric but presumably post-Pleistocene mammoths, frozen into the arctic permafrost and preserved by natural refrigeration.
Important as these unusual localities are, most of the fossil record is composed of skeletons that can be recovered from ordinary sediment, in a cliff or quarry or roadcut, or from wells drilled deep into the ground. These skeletons and skeletal fragments may be preserved as the original material, as hollows (molds) formed by the dissolution of the original matter to leave only an imprint, or as replacement material, in which instance a mineral such as quartz or pyrite has replaced the original bone or shell.
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. 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 1800s, 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, but the development of radiometric age-dating methods has enabled scientists to calibrate Earth history in actual years.