How Are Fossils Formed?
How are fossils formed is the question that comes to mind when we see magnificent specimens like the one to the right. There are many ways fossils can form from. These include freezing, being submerged in a tar pit or becoming trapped in amber (tree sap). But by far the most common method is when an animal or plant sinks to the bottom of a body of water when it dies and becomes covered with silt. This method of fossil forming was responsible for what is perhaps the best known fossil of them all, that of the ancient proto-bird Archaeopteryx. So how are fossils formed?First the animal (or plant) must die over or in water. In the case of Archaeopteryx, one possibility is that the creature was flying over a lake that was giving off poisonous gases. This is backed up by the presence of other flying organisms in the same fossil deposit. If so, Archaeopteryx would have dropped like a stone into the lake, which accounts for the intact condition of the body.
Once at the bottom of the lake the animal is covered by a layer of silt or mud. This does two things. First, it stops scavengers eating the body, which would break up the body and scatter the bones. Secondly, it means the body will decay in an anaerobic environment, meaning there is no oxygen. This slows down the rotting process and allows more details of the animal to be preserved.
Once the animal is under the silt, more silt and mud are laid down over the years. Centuries pass, and the weight of all the silt compresses the lower layers. This drives out water and eventually causes the silt to become siltstone, sandstone or similar. During this process the bones and sometimes softer parts are replaced with minerals from the surrounding rocks, effectively turning the creature to stone.
Then geological ages pass as the earth's crust moves. Eventually the siltstone that was once the bottom of the lake may be pushed up and exposed, allowing some lucky fossil hunter to find the now ancient remains of the animal.This process has been going on as long as there have been animals and plants. It is also a process that is continuing today, even as you read this. However, the fossils that are forming at the bottom of today's lakes and oceans are unlikely to ever be seen by any human eye. Return from How Are Fossils Formed to General Evolution or Return to the Reptiles Home Page
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