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A fossil can be any evidence of past life, such as preserved shell or bone, petrified wood, borings or footprints. The fossilization process requires quick and undisturbed burial. Most fossils are found in sedimentary rocks, but in rare cases whole animals have been preserved in frozen ground, ice, asphalt, amber or in arid conditions. Usually fossils are the hard parts of animals such as teeth, bones and shells and wood--some preserved with little or no alteration. Fossils of soft parts are rare but can be found in amber, the hardened resin of ancient trees, and in shale where plants and marine animals have left behind a thin layer of carbon that outlines their structure. Even sandstone casts or imprints of dinosaur skin have been found. Fossils of hard parts are often alterations. As water dissolves chemicals in the hard parts other chemicals or compounds replace them. Silica, lime and iron are common replacements. The replacement can preserve the original sturcture of the plant or animal. The silica replacement in some petrified wood shows the plant cells and annual rings. In most replacements, however, only the general form is preserved. Some fossils are only indications of life. When all the original material is dissolved and only a cavity remains, it forms a mold or imprint in the rock. When the mold is filled in with new material such as mud, a cast of the original is formed. These are among the most common fossils found in sedimentary rocks. Other types of fossils that are evidence of ancient life are; borings of worms and mollusks, gastroliths which are smooth, rounded rocks found in dinosaurs, corprolites or fossil excreta and artifacts such as tools and weapons made by ancient man. Collenia, a Pre-Cambrian limy alga is the oldest known fossil.