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Extreme Cold
Extreme cold is one way a fossil is preserved. Freezing is one of the rarest forms of fossilization because the conditions need to be just right. An animal typically falls into a pit or crevasse where it is frozen solid until it is discovered. This form of fossilization is ideal because it can preserve bone, skin, muscles and internal organs. Some animals that were flash frozen have been discovered with food still in their mouths and stomachs.
Dry Air
The dry air in arid desert regions can help make a fossil. Drying is a form of preservation, also known as desiccation. The dry air removes all the moisture from an organism, thus preserving it, as moisture is a key element in decomposition. Dry air can mummify and preserve soft tissues including skin and organs.
Tar
Tar, also known as asphalt, is an excellent preservative. Tar comes from crude oil that seeps through fissures in the earth's surface. As the lighter elements in the oil evaporate, a dense, sticky, tar-like substance is left behind. This substance can make fossils by preserving bones, teeth, shells, exoskeletons and plant seeds. The La Brea Tar Pits in Los Angeles are a prime example of how tar makes fossils, as many fossilized remains have been found in and around the area.
Tree Sap
Tree sap is polymerized and converted into amber. Many fossilized insects have been found encased in the amber. Just as described in the movie "Jurassic Park," an insect lands on a tree branch and becomes caught in sap. Once the sap with the insect inside is polymerized and converted to amber, you have the fossilized remains of a prehistoric insect.
Decomposition
Although decomposition is typically associated with the recycling of organic materials, occasionally decomposition can lead to carbonization, thus making a fossil. Carbonization occurs when a plant or animal decomposes and leaves only carbon behind. The carbon then creates an impression in a rock that shows the outline of the fossil.
Petrification
Petrification can occur one of two ways to make a fossil. The organism can be fossilized through a process called replacement, in which water dissolves any hard parts and replaces them with mineral matter. This process happens very slowly over time; the most common replacement minerals are calcite, silica, pyrite and hematite. Permineralization is the second way petrification can occur. During permineralization, water carries minerals that infiltrate the organism's microscopic pores and cavities, producing stone-like fossils.