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A fossil (from Classical Latin fossilis ; literally, "obtained by digging")  is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons , stone imprints of animals or microbes , hair, petrified wood , oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.
There are many processes that lead to fossilization, including permineralization , casts and molds, authigenic mineralization, replacement and recrystalization, adpression, carbonization , and bioimmuration.
In some cases the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.