Fossils are the remains of creatures which existed long ago. Fossils range from thousands of years to many millions of years in age. The earliest fossils of complex life date from around 600 million years ago, however recent reports reveal bacteria may have existed up to 3 billion years earlier. To put this in context, the dinosaurs became extinct just 65 million years ago!
Not all former life was preserved as fossils, in fact the vast majority simply vanished without trace. The most likely materials to survive fossilisation are the hard parts such as shells and objects which in life were constructed from resistant materials, such as Coral. In order for softer materials to survive, the conditions must be extremely favourable.
Fossils come in a variety of sizes, from minute traces to large skeletons. Trace fossils are clues to former life, they result from the activities or presence of creatures and plants. Examples of these traces include footprints, burrows and root tunnels. At the larger end of the scale, fossils also include bones, the largest of which belong to the dinosaurs, which existed between the Triassic and Cretaceous periods.
There are five different ways ways an organism can become fossilised:
Permineralization (Petrification) - This process involves the replacement of the original organic tissues with minerals from the surrounding rock, including silica, calcite or pyrite.
Unaltered preservation - This occurs when the organism is preserved in its original state and protected from the affects of permineralization. Examples of this include insects which become trapped in tree sap, which later turns to amber.
Carbonization (Coalification) - This results from removal of all but the carbon elements. Other elements such as hydrogen, oxygen, and nitrogen are removed.
Authigenic preservation - These fossils are the molds and casts of organisms which have dissolved or rotted away, leaving only a trace of their existence.
Recrystalization - This occurs where crystals form within the original structure, eventually replacing it and resulting in a crystallized copy. The following diagrams illustrate the process of fossilisation, starting with the moment of death. The example used is an ammonite, a shelled creature that lived in the seas around 150 million years ago.