• Some animals were quickly buried after their death (by sinking in mud, being buried in a sand storm, etc.).
• Over time, more and more sediment covered the remains.
• The parts of the animals that didn't rot (usually the harder parts likes bones and teeth) were encased in the newly-formed sediment.
• In the right circumstances (no scavengers, quick burial, not much weathering), parts of the animal turned into fossils over time.
• After a long time, the chemicals in the buried animals' bodies underwent a series of changes. As the bone slowly decayed, water infused with minerals seeped into the bone and replaced the chemicals in the bone with rock-like minerals. The process of fossilization involves the dissolving and replacement of the original minerals in the object with other minerals (and/or permineralization, the filling up of spaces in fossils with minerals, and/or recrystallization in which a mineral crystal changes its form).
• This process results in a heavy, rock-like copy of the original object - a fossil. The fossil has the same shape as the original object, but is chemically more like a rock! Some of the original hydroxy-apatite (a major bone consitiuent) remains, although it is saturated with silica (rock).

• unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap)
• permineralization=petrification (in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized)
• replacement (An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron)
• carbonization=coalification (in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed)
• recrystallization (hard parts either revert to more stable minerals or small crystals turn into larger crystals)
• authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).

Also, some fossils of skin (and other soft body parts) have been found. Again, the color of the skin is not retained during the fossilization process, all that remains today is a rocky model of the original.

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