So a fossil may be either a cast or a mould. This series of events rarely happens on land, so fossils of sea creatures are more common than those of land animals. The fossil record When an animal dies, its soft parts rot away, but if its bones or shell are buried quickly in mud, they may eventually turn to stone, creating a fossil. As the next pages show, fossils give a remarkable picture of the history of life. We know from fossils when creatures first began to crawl ashore from the sea, what the dinosaurs were like, and much more besides.
Most fossils are either shells or just a few isolated bones.
Complete skeletons are rare. In fact, fossils show that animal species alive today are only a tiny fraction of those that have lived on the Earth. Over time, many millions of species have evolved and then died out, leaving only their fossils to prove they ever existed. You can work out how old a fossil is from the rocks in which it is found. Scientists know how old rocks are relative to each other because layers of rock form on top of each other, so the lowest layers are oldest.