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Ways a Fossil Can Be Made

Bones, imprints or other traces of once-living organisms are called fossils. In many cases, these incomplete remains serve as the only clues scientists have available to reconstruct what life looked like in earlier periods of Earth's history. Depending on the organism and its environment, fossils form in a variety of different ways. Ultimately, the formation of a fossil is a very rare event that only occurs under certain conditions.

Permineralization
- Sometimes bones, shells or plant remains become saturated with ground water. As the water evaporates, it leaves behind deposits of minerals like calcite. These accumulating deposits gradually fill the pores of the organism's remains, taking the place of the organic matter. The end result preserves the organism's shape even though its composition has been lost. A more extreme version of this same process is petrification, through which ancient wood is gradually "turned to stone."
Compression
- Swift burial beneath sediment can help protect an organism's remains from the bacteria that cause decay. As the sediment accumulates, the remains become compressed through the course of long ages, gradually removing the hydrogen, oxygen and nitrogen the organism once contained. The result is a thin carbon film that preserves some details of the organism's outline and shape. In other cases, the organism may be dissolved by carbonated ground water that penetrates through the rock. This process creates a hollow mold of the original organism's bones or other remains. Alternatively, mineral deposits may fill the cavity to form a kind of cast that preserves some details of the organism's shape.
Freezing
- In some very rare cases, an organism may be preserved intact by being frozen whole so that both bones and soft tissue are preserved. These rare incidents probably occur when an animal like a woolly mammoth stumbles into a melted ice crevice and is entombed. Fossilization in amber is another rare case where organisms are occasionally preserved intact. Sometimes insects or other small organisms become trapped in sticky tree sap; additional layers of resin may build up atop its remains. As the resin hardens it turns into amber -- a golden tomb with an insect trapped inside.
Other Fossils
- In deserts and other extremely arid environments, dead organisms can become mummified and preserved almost intact for millennia if not more. These environments are so dry the organism's remains become completely dried out or desiccated. The bacteria that cause decay need moisture to grow and thus can find no foothold on this kind of corpse. Occasionally the tracks, traces or footprints of an animal walking through mud may also become fossilized. If the impression quickly fills with sand and the right conditions are present, the traces may eventually become immortalized in sandstone.

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