Text lesson

Unit 4 — Dating Planet Earth

Science — Third Preparatory

1.2 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:

Key points

Sedimentsfragments of clay, sand or limestone that bury the remains.

Permafrost (ice)ground frozen for very long periods without melting. (Mammoths have been found preserved in ice with skin and hair.)

Natural tara form of crude oil that traps and preserves animals.

Amberhardened 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.

Unit 4 — Dating Planet Earth · 3 / 28

1.2 What is a fossil and how does it form?