Animal fossils
We know about animals that became extinct millions of years ago because scientists have found their fossils. A fossil is an animal's skeleton preserved in rock. This happens when its body becomes covered in mud. The mud gradually buries the skeleton and turns into rock around it. Millions of years later it is discovered when coal is being mined or when the sea washes part of a cliff away.
By studying fossils, scientists discover what extinct animals looked like and how they lived. They can tell the age of a fossil by measuring how deep it is buried, and by studying the layers of rocks around it.
Very few animals become fossilized, because usually their bodies are eaten by other animals or else their bones decompose. Sometimes only parts of a skeleton become fossilized. Some ani¬mals are well preserved; every detail of the bones is seen, sometimes with the outline of the body.