2 What is a fossil and how does it form?

Fossils are the remains or traces of organisms (plants, animals, micro-organisms) that lived in ancient geologic times and were preserved because of special environmental conditions after they died.

In rare cases, a whole organism is preserved.

• Fossils usually form when the organism is buried quickly after death in a medium that protects it from decay (decomposition by bacteria and fungi, and scavengers).
• Suitable media include:

1. Sediments: fragments of clay, sand or limestone that bury the remains.
2. Permafrost (ice): ground frozen for very long periods without melting. (Mammoths have been found preserved in ice with skin and hair.)
3. Natural tar: a form of crude oil that traps and preserves animals.
4. Amber: hardened resin, the sticky substance secreted by some trees, which can trap insects perfectly.

• Why are fossils of soft-bodied organisms rare? Soft tissues (such as jellyfish, worms, skin, muscles) decay quickly after death, before burial can protect them.
• Hard parts (bones, teeth, shells) last longer, so most fossils are of hard parts.