The most common way fossils form begins as soon as a plant or animal dies. As the organism rests on the ground—or on the floor of a sea, lake, or river—it is gradually buried by sediment, often carried in by water. In the diagram above, an ammonite dies in a shallow sea and, over time, layers of sediment cover it (as shown in the second diagram). The ammonite’s soft tissues decay, while minerals dissolved in the surrounding water seep into the shell and replace its original material—a process known as permineralization. Eventually, after hundreds of thousands to millions of years, uplift and erosion expose the rock layers, and the fossil becomes visible at the surface.

Of all the prehistoric life that has ever existed, only a tiny fraction survived the fossilisation process. In most cases, the conditions at the time an organism died were simply not suitable for preservation.

Most fossils are found in sedimentary rocks, which form from the build-up of sediments in rivers, lakes, and seas. The majority of animal and plant fossils discovered today originally lived in or near these environments. After death, their remains were often broken up and deposited on the beds of rivers, lakes, or seas, where they were gradually buried by layers of sediment. As these layers built up, the increasing weight caused many remains to be compressed or crushed. Over time, the sediments compacted and hardened into rock.

Across millions of years, these rocks shifted, moved, and were eventually exposed to the elements. As erosion wears away the surrounding rock—or as stone is quarried—the fossils are revealed and can be discovered and collected.

The best conditions for fossilisation

• Rapid burial in moist sediment, which protects remains from scavengers and slows decay by bacteria.
• Quick burial by volcanic ash. Many dinosaur fossils in the American West and China were preserved this way.
• The presence of hard body or plant parts, such as teeth, bones, shells, or wood, which are more resistant to decay.
• Stable, unchanging temperatures, which reduce physical and chemical breakdown.
• Groundwater that is rich in minerals, allowing minerals to replace or infill the original material.
• Very fine sediments, which bury remains more effectively than coarse gravels.
• Calm environmental conditions, so remains are not damaged or scattered by waves or strong currents.

Other types of preservation

Amber

Sticky resin from trees and plants sometimes trapped insects, spiders, and small animals such as frogs. As the resin hardened and fossilised into amber, it sealed off the organisms from air and decay, resulting in exceptionally well-preserved fossils that often retain fine details of the trapped animals.

Mummification

A small number of mummified animal remains have been discovered in caves, where dry, sterile conditions slow decay. In most cases, only the bones are preserved, but in rare instances skin and other soft tissues can survive. Mummification is not true fossilisation; instead, it represents a temporary halt in the normal process of decomposition.

Freezing

Animals and plants can be exceptionally well preserved when trapped in ice. Mammoths, for example, are often found in Siberia with their skin and hair still intact. Remains frozen in ice or permafrost can provide a wealth of information about the animal’s biology, diet, and the environment in which it lived.

Tar

Insects and animals have also been found preserved in tar. While tar can effectively slow decay and protect remains, this type of preservation is only stable for thousands of years, rather than the millions of years typical of true fossilisation.

Pyritisation

During fossilisation, some remains can be replaced by iron pyrite. Fossils preserved in this way are unstable when exposed to moist air, so they must be stored in very dry conditions to prevent deterioration.

Silicification

Wood is often found silicified in freshwater environments and in terrestrial sand and silt deposits. Silica released from the weathering of volcanic ash slowly infiltrates partially decayed wood, gradually replacing its original structure and preserving it as stone.