Since life began, parts of animals have been preserved as fossils. When the animals died their remains were covered with mud and sediment that slowly turned to rock. When scientists examine layers of ancient rock, the oldest layers are at the bottom, and new rock is laid down on top of them. So unless the rocks have been disturbed by movements of the Earth’s crust, the deeper the fossil is found then the older it is.
By studying rocks and the fossils they contain, it is possible to decide how old fossils are, and to identify creatures that lived at the same time but in different parts of the world.
Usually only the hard parts of extinct animals are fossilized, but recently some fossils have been found showing internal organs, and there are even fossils of jellyfish.
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