Fossils are the remains of living things preserved for millions of years, usually in stone.
Most fossils are the remains of living things such as bones, shells, eggs, leaves and seeds.
Trace fossils are fossils of signs left behind by creatures, such as footprints and scratch marks.
Paleontologists (scientists who study fossils) tell the age of a fossil from the rock layer in which it is found. Also, they measure how the rock has changed radioactively since it was formed (radiocarbon dating).
The oldest fossils are called stromatolites. They are fossils of big, pizza-like colonies of microscopic bacteria over 3500 million years old.
Scientists study fossils to learn about the Earth’s history and about the animals and plants that lived millions of years ago.
When an animal dies, its soft parts rot away quickly. If its bones or shell are buried quickly in mud, they may turn to stone. When a shellfish such as this ancient trilobite dies and sinks to the sea-bed, its shell is buried. Over millions of years, water trickling through the mud may dissolve the shell, but minerals in the water fill its place to make a perfect cast.
The biggest fossils are conyphytons, 2000-million-year-old stromatolites over 100 m high.
Not all fossils are stone. Mammoths have been preserved by being frozen in the permafrost (see cold landscapes) of Siberia.
Insects have been preserved in amber, the solidified sap of ancient trees.
Certain widespread, short-lived fossils are very useful for dating rock layers. These are known as index fossils.
Index fossils include ancient shellfish such as trilobites, graptolites, crinoids, belemnites, ammonites and brachiopods.