How are Fossils Formed

Fossils are amazing time capsules holding information that can be read from an era spanning many millions of years.

By the time we are able to hold this excavated jewel in our hands, the sample has undergone a complicated process that basically turns the biological material into something that can be likened to ordinary rock.

So how exactly is a fossil formed? How is it that we can still see the perfectly preserved indentations or skeleton of a creature that has been dead from such a long time ago?

The secret is in the location of the original deposit and a lot of luck. The conditions have to be just right for preservation to take place as most animals usually just disappeared, naturally decomposing and leaving no trace.

An organism can be fossilized in a number of ways:

Permineralization (Petrification) An organism is immediately covered in sediment shortly after death. Many such fossils found are marine organisms which were covered in oceanic sediment. The actual process of permineralization is very gradual and occurs when water seeps through the sediment that is covering the organism. It fills every fold of the body and helps to keep the specimen in one piece whilst the mineralized water slowly soaks into the remains.

Unaltered preservation – This occurs in younger types of sedimentary rock and is typified by an organism preserved in its original state and protected from the affects of permineralization. Examples of this type of fossil, such as the perfectly preserved remains of an insect can often be found in samples of amber, (originally tree sap)

Carbonization (Coalification) – These fossils are produced from plant leaves, and some soft body parts of fish, reptiles, and marine invertebrates that have decomposed and left the carbon behind. These are very dark in color and often the fossil is very detailed. Hydrogen, oxygen, and nitrogen have also been removed.

Authigenic preservation – These fossils are the molds and casts of animals or organisms which have rotted away. Some examples of this can be found of sea shells, bivalves (scallops & mussels), sponges and echinoids (sea urchins)

Recrystalization A good example of this process is the ammonite which existed 150 million years ago. Recrystalization changes the internal crystal structure of a fossil and converting the original minerals into larger crystals. Many shells for example which were originally made of calcium carbonate (aragonite) will be altered into a type of calcium carbonate called calcite.

Drying or desiccation is another form of fossilization. The process is known as mummification and is usually found in really dry parts of the world. Tissue and organs can be completely preserved if they have been allowed to become totally desiccated.

Frozen fossils are found from the period of the Ice Age and include animals such the mammoth and wooly rhinoceros. Found preserved in ice from Alaska and Siberia, the preservation of such specimens is often so complete that it will leave the flesh, skin, and hair intact, and suggests that the animal was flash frozen, often being found with food still in the mouth!


Discovering Fossils.
(Accessed 25/03/2009)

University of Arizona Geosciences 308 Paleontology Laboratory Manual
(Accessed 25/03/2009)

Fossil Formation
(Accessed 25/03/2009)