Fossils are the remains of ancient organisms, usually animals and plants but also minute bacteria and occasionally fungi. They are preserved by natural processes of burial. Most are entombed in sediment - mud, silt or sand - which with time transforms into a sedimentary rock such as sandstone. More rarely, fossils are enclosed in ash erupted from volcanoes, resin that oozes from trees and hardens into amber, or natural tar seeping from the ground, as at the famous La Brea site in Los Angeles.
Normally only the resistant hard parts of organisms are fossilized. In the case of animals, these are mostly shells, bones and teeth, all of which contain biominerals secreted by the living animals. Only in rare instances of exceptional preservation are soft, fleshy tissues fossilized and even these never retain their original compositions.
Following burial, organic remains undergo various physical and chemical changes. They may be crushed or dissolved, their chemical composition can alter subtly or markedly, and any holes in the structure may become filled with minerals precipitated from solutions that percolate through the rocks. The 'biological signature' of the animal becomes progressively vaguer, especially if the fossil is deeply buried and subjected to high temperatures and pressures.
The oldest known fossils date back some 3.5 billion years but the majority of fossils, and almost all non-microscopic forms, are younger than 550 million years old.
How are fossils formed?