by A W Drake

Created on: March 28, 2009   Last Updated: April 04, 2009

In the popular imagination fossils evoke images of huge dinosaur relics encased in stone but a fossil can be anything from micro organisms (the most common type of fossil by far), to foliage, trees, insects and animals of all sizes. Other than preserved animal remains there are also fossils of animal activity known as trace fossils. These can include footprints, track ways, burrows, bites and droppings (coprolites).

For about 3000 million years the only life on Earth was to be found in the sea, and it was not until 430 million years ago that plants and animals appeared on land. For this reason the oldest fossil types are marine creatures. Considering all the organisms that have lived within this huge time span fossil creation is very rare, relying on specific conditions to prevent the natural breakdown of organic remains and the process of the fossil to form.

After death the remains of most organisms last for only a relatively short period due to decomposition, scavenging by other animals and the effects of weathering, so the conditions that allow fossil formation need to be present very soon after the organism dies.

Permineralization

The most common process of fossilization is known as
permineralization and occurs when the organism dies and is covered by water. Ideal sources of water are a river bed or a delta. The water protects the animal from the elements and may contribute to the decomposition of the soft tissue by bacteria, leaving just the skeleton or shell.

Sediments in the water now bury the remains. The sooner this happens the more likely a fossil will form. Mud slides at a river delta can provide a quick build up of sediment and further protection from decomposition. As the layers of sediment continue to build up they eventually compress the lower levels into rock (a process known as lithification) which will encase the fossil.

The type of sediment the specimen is buried in will affect the quality of the fossil. For instance, if the sediment is sandy the fossil may have poor detail, but fine sediment will give the fossil much greater resolution.

The chemical composition of the sediment will also be influential. Iron in the sediment can give the fossil a red appearance, whilst phosphorous can darken it to grey or black.

If the water is especially rich in minerals they will percolate through the layers of lithified sediment giving an even greater chance for the fossil to form. Many types of organic material such as bone, wood and shell have empty spaces, filled

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