Discovering Fossils 05-30

How do fossils form?
The term 'fossilisation' refers to a variety of often complex processes that enable the preservation of organic remains within the geological record. It frequently includes the following conditions: rapid and permanent burial/entombment - protecting the specimen from environmental or biological disturbance; oxygen deprivation - limiting the extent of decay and also biological activity/scavenging; continued sediment accumulation as opposed to an eroding surface - ensuring the organism remains buried in the long-term; and the absence of excessive heating or compression which might otherwise destroy it.
Fossil evidence is typically preserved within sediments deposited beneath water, partly because the conditions outlined above occur more frequently in these environments, and also because the majority of the Earth's surface is covered by water (70%+). Even fossils derived from land, including dinosaur bones and organisms preserved within amber (fossilised tree resin) were ultimately preserved in sediments deposited beneath water i.e. in wetlands, lakes, rivers, estuaries or swept out to sea.
Fossilisation can also occur on land, albeit to a far lesser extent, and includes (for example) specimens that have undergone mummification in the sterile atmosphere of a cave or desert. However in reality these examples are only a delay to decomposition rather than a lasting mode of fossilisation and specimens require permanent storage in a climate controlled environment in order to limit its affects.
In the following example a fish is used to illustrate the stages associated with fossilisation within off-shore marine sediments. This is just one summarised example, in reality there are countless scenarios that create the conditions necessary for fossilisation in marine sediments.
Death
Having reached adulthood and returned to its birth place to spawn, this particular fish reaches the end of its life and dies. Soon after death the body of the fish becomes water-logged and sinks to the seafloor (note that quite often the gases produced during decomposition cause the carcass to float back to the surface, so the final resting place may be some distance away). More often than not the carcass would be pulled apart and scattered by scavenging crustaceans and other fish, however on this occasion the absence of any large scavengers leaves the fish relatively undisturbed.