The most common way fossils are formed starts as soon as an animal or plant dies. As the organism lies on the ground, or on the bed of the sea or lake, it is covered by sediment. This sediment is usually brought by water. In the diagram above, the ammonite died in a shallow sea environment and, over time, sediments covered it (shown in the second diagram). The soft body of the ammonite decayed and minerals in the water replaced the original minerals in the shell (that is, by permamineralization). Finally, after hundreds of thousands (or millions of years), the land is eroded and the fossil can be seen.
Of all the prehistoric life that died, it is only a tiny amount that has survived the fossilisation process. The conditions when the majority of organisms died were just not right at that time to preserve them.
Most fossils are found in sedimentary rocks, which were formed from the sediments of rivers, lakes and seas. The majority of the animal and plant fossils we find today originally died near or in these environments, got broken up and were deposited on the beds of rivers, lakes and seas. The sediments covered them and, over time, some of the layers grew so thick that many of them got crushed. The sediments compacted and over time and turned to rock. The rocks shifted, moved and became exposed to the elements. This process can take millions of years and, now, as the rocks erode or are quarried, the fossils become exposed and can be collected.
The best conditions for fossilisation
- The quick burial of animal remains in moist sediments. This prevents scavengers from eating and bacteria from decaying them.
- The quick burial in volcanic ash. Many dinosaur bones in the American West and China have been found buried in volcanic ash.
- The presence of hard body or plant parts (for example teeth, bones, shell and wood).
- Unchanging temperature conditions.
- Ground water that is heavily mineralised.
- Sediments that are very fine make for better burial than coarser gravels.
- Calm conditions, so that remains are not broken up (for example, by wave or current action.)
Other types of preservation
The resin from trees and plants, while still sticky, sometimes trapped insects, spiders and small animals, such as frogs. This resin hardened to become amber, perfectly preserving the encased animal.
A few mummified remains of animals have been found in caves, where the conditions are dry and sterile. Usually, only the bones are preserved in this way, but occasionally, skin and other tissue can be preserved. Mummification is not true fossilisation, just a pause in the disintegration process.
Animals and plants can be extremely well preserved when preserved in ice. Mammoths are often found in Siberia, with skin and hair intact. Fossils frozen in ice and permafrost can give us a great deal of information on the life and habitat of the animal.
Insects and animals have been found embalmed in tar. Tar preservation can only remain stable for thousands of years, not millions.
During fossilisation, some fossils can be replaced by iron pyrite. However, fossils preserved this way become unstable when exposed to moist air, so they need to be kept in very dry conditions.
Wood is often found silicified in freshwater, and in terrestrial sand and silt deposits. Weathering volcanic ash supplies the silica, which gradually incorporates into the partially decayed wood.