Fossils are the remains or traces of an animal or plant that are preserved in rock. They come in many forms, from footprints and faint impressions of leaves to shells and bones of animals. Fossils tell us about the life of the past, and when different species evolved and died out. They are also a guide to the age of the rocks in which they are found. Animals and plants are FOSSILIZED when they are buried in sedimentary deposits that are hardened and compressed into sedimentary rocks.
It is rare for the actual body parts of an animal or plant to remain intact when it is fossilized. Sometimes the parts are mineralized (replaced by minerals), or they may rot away, leaving a cavity called a mould. Sometimes the mould fills with rock to form a cast. A trace fossil is a fossil of a mark made by an animal, such as a footprint.
William Smith is known as the father of British geology. While working as a surveyor, Smith noticed that layers of sedimentary rocks (known as strata) were laid down in a particular order and that the layers could be matched by the fossils they contained. This enabled him to draw the first proper geological map of England.
The process that turns the remains of an animal or plant into rock is called fossilization. It takes many thousands or millions of years. The remains become fossils only if they are buried by sediments before they rot away. This can happen at the bottom of lakes and seas or when dead animals beside a muddy river are quickly covered by floodwater.
When a fish dies, its body falls to the bottom of the sea or a lake. Its soft parts are eaten by other animals, leaving just a skeleton.
A layer of sediment settles over the skeleton, which becomes buried. More sediment settles on top, burying the skeleton deeper.
The buried sediment slowly turns to sedimentary rock. The fish skeleton is partly or wholly replaced by minerals that preserve its shape.