| Themes > Science > Paleontology / Paleozoology > Fossils And Fossilisation > How and Where Fossils Form |
The Earth is teeming with life, all of it searching for food--organic compounds of hydrogen and carbon. 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; sponge spicules, diatom frustules, and radiolarian tests are composed of opaline silica. 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. Although few such occurrences are preserved and discovered, they 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 and crustaceans of the Cambrian Period. At the Carboniferous Mazon Creek locality in Illinois, both land plants and marine invertebrates are preserved. The Holzmaden oil shale (see Shale, Oil) of southern Germany, of Early Jurassic age, is well known for its many fish and crinoids (see Crinoid), as well as for its ichthyosaurs and other marine reptiles that have been found there and displayed in museums throughout the world. 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 (pterodactyls) and the world's earliest known bird, 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, Kansas. 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 faunas, from vultures to bisons, wolves to saber-toothed cats to beetles, has come from tar seeps, such as the La Brea Tar Pit in Los Angeles, Calif., 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 (see Mammoth), 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 dissolving the original matter to leave only an imprint, or as replacement material, with a mineral such as quartz or pyrite having replaced the original bone or shell. |
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