Fossils are what is left of plants and animals. They are from long ago. Fossils are found in rock. Bones can become fossils. Shells and leaves can become fossils too. There are two kinds of fossils. They are body fossils and trace fossils. Feather or skin prints can be trace fossils. So can footprints.

Most animals and plants that die rot away. They do not become fossils. A dead animal or plant has to be covered up fast. Mud, sand or silt often bury it before it rots. Bones, teeth and shells are hard parts. Hard parts become fossils more often than soft parts.

Water full of minerals seeps into buried bones and teeth. It slowly turns them to stone. Layers of mud and sand pile up. They slowly turn into rock. Making a fossil takes a very long time. It is often thousands to millions of years.

Amber is tree sap that turned hard and old. Amber can hold tiny animals such as ants and flies. Petrified wood is wood that has turned to stone. Trilobites are very common fossils. They are the most common arthropod fossil in national parks.

Fossils tell us which plants and animals lived long ago. We know dinosaurs lived because we find their fossils. Tiktaalik was a fish. It had a flat head like a crocodile. Pakicetus was an early whale. It lived on land.

Words to know

• silt: fine mud
• minerals: rock bits
• petrified: turned to stone