Fossil Formation
Fossil Formation
The fossil record is a key source of evidence that helps scientists understand life's history. Fossils are the remains, impressions or traces of animals and plants from the remote past. Fossils usually take the form of a mold or a cast in rock. Generally, a preserved specimen is regarded as a fossil if it is older than approximately \(\text{10 000}\) years (although this is not a strict definition).
Examples of common fossils are skeletons or parts of skeletons, shells or teeth. Sometimes plants or animals can leave imprints that get preserved as fossils.
The study of fossils across geological time, how they are formed, and how ancient organisms have evolved in relation to other phylogenetic groups, is called palaeontology.
How fossils are formed
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For fossilisation to occur, a plant or animal must first die. Soft tissues decay quite quickly, therefore animals that have hard exoskeletons and woody plants tend to fossilise better than soft-bodied organisms.
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The organism (plant or animal) must be buried beneath mud and or soil shortly after death. Although decay still takes place, the lack of oxygen slows it down. As more and more layers of mud and soil are added, the sediments become compressed.
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Eventually, this compression turns the sediment into rock, which forms a mould around the shape of the original skeleton. Sometimes the original bone or shell softens and dissolves completely, sometimes the bone or shell remains. Water that is rich in dissolved minerals trickles in through the layers of sediment into the mould.
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The mineral-rich water enters the hollow and crystallises to create a natural cast of the original organism. Otherwise, the minerals slowly seep into the skeleton, changing its chemical composition and making it capable of surviving for a long time.
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Over many millions of years, rock that was once buried rises again to the surface and is eroded away, exposing the fossils.
Fact:
If the rocks surrounding a fossil are distorted and squeezed by geological forces, it will result in distortion of the fossils within them.
Fossilisation is very rare and only happens when a plant or animal dies in exactly the right circumstances. Usually animal corpses are eaten by other animals or decomposed by bacteria before fossilisation can occur. Even hard parts, like bones and shells, are eventually destroyed through erosion and corrosion.
Optiona Video: Walking With Lucy | California Academy of Sciences
It takes a rare set of circumstances to turn a living creature into fossilised bone. In the case of Lucy, the famous hominid fossil discovered in Ethiopia's Great Rift Valley in 1974, there is no evidence that she met a violent death. No predator or scavenger found her body before it began rotting in the lake's soft sediments. Her bones, which settled in the mud, may have been cracked or shattered by animals roaming around the shore.
Heavy rains gradually washed in enough sand and gravel to bury her bones. These deposits built up over thousands of years, burying her remains hundreds of metres deep. The calcium in her bones, molecule by molecule, was replaced by minerals from these deposits, turning the bones to stone.
She remained buried over millions of years, while the Earth's crust moved constantly, forcing the remains of her body closer to the surface. Heavy storms beating down on the earth eroded the sediment and most likely brought her once again to the Earth's surface. Her exposure made it possible for anthropologists to later discover her remains some three million years after her death.
This lesson is part of:
History of Life on Earth