Fossils come from sea creatures, dinosaurs, plants and other ancient animals. Over 250,000 different kinds of fossils have been identified. After the death of an organism, the soft tissue is ordinarily consumed by scavengers and bacteria. The empty shell or bone is left behind to petrify within the gradually hardening sediment covering the animal part.
Sedimentary rocks start out soft and squishy ("unconsolidated"). Fossils form when animals or plants die in the unconsolidated sediments and are covered by more layers. Sediments can become hard over time if exposed to higher temperatures and pressures or certain minerals that cement the grains together.
Fossils are the organism's parts replaced by minerals that were dissolved in water. So, the fossils we see are really mineral deposits, not the real body parts. Fossils can be the remains of plants and animals, or traces of their lives such as footprint. Sometimes the original animal or plant material is preserved, other times only an imprint or mold is preserved, and sometimes the original material is replaced by new material in the same shape as the original.
Fossils are found in sedimentary rocks, but almost never in other rock types. You need to start out with a soft material to make an imprint. Igneous rocks are very hot when they are soft/molten, so they burn up organic materia. Metamorphic rocks are exposed to such intense heat and pressure that any fossils are destroyed.
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