Fossils are the remains of plants or animals from prehistoric times. They are a rarity as most organisms, then and now, are either consumed by other organisms or completely decay at death. Fossil remains are preserved in a variety of ways.
One method of fossilization is petrifaction. This is when the organic plant or animal material is replaced with minerals and eventually hardens into rock. Examples of this are the petrified trees found in the Petrified Forest National Park in Arizona.
Entire organisms have been found encased in amber, which is a gold-colored resin that formed from pine tree sap. Ants, pollen grains, bees and other organisms have been found in this resin.
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In certain parts of the world, entire animals have been preserved in ice. In Siberia and in the upper reaches of North America, scientists have found Mammoth fossils complete with hair, skin and internal organs.
Sometimes an organism dies and is very quickly buried. The organism is compressed between rock faces by the Earth’s subsurface pressure. The organism does decompose, but leaves behind a carbon imprint of itself on the rock face. Plants are most often preserved in this fashion, but insects, fish and other animals have been found.
Sedimentary rock is produced by sediments such as mud or sand, usually found on rivers, lakes, estuaries and ocean bottoms. Most fossil remains are preserved, and found, in sedimentary rock. This makes fossils of sea organisms more common than their land-based counterparts.