Fossils are any trace or remains of plants and animals that lived in the prehistoric past . The vast majority of fossils are found in sedimentary rocks- shale, limestone and sandstone where organisms were buried as sediments accumulated. Quick burial in sediments usually under water is the commonest way for fossilization. The fleshy parts of the buried creatures rot away quickly but the bones or shells remain. Sometimes these hard structures survive virtually intact and at other times their pores and open spaces are partially or entirely impregnated with mineral deposits from seeping groundwater and the structures are turned into stone.
In another type of fossilization, the original structure completely dissolves away, leaving an opening or natural mold in the rock. The mold retains the original shape of the original. Molds of leaves and other very thin structures are known as imprints. Leaves and soft bodied animals can also be preserved as films of carbon that form silhouettes of the originals.
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