How do fossils form?
Fossils are the preserved remains or traces of ancient life. They can be bones, teeth, eggs, footprints or the impressions of feathers. A fossil can form in several ways. For dinosaur bones, two things often matter most: being buried in sediment, and water rich in minerals.
Burial protects the remains
When an animal dies, its soft parts usually rot away or get eaten. Bones and teeth can last longer, but they too may be broken up or carried off.
A covering of sand, mud or other sediment can protect what is left. The faster the remains are buried, the better their chances, though even then nothing is guaranteed.
What minerals do
Water can carry dissolved minerals into the tiny spaces inside a buried bone. Crystals grow there and help to preserve the bone’s structure. Minerals may also take the place of some of the original material.
Sometimes a bone or shell dissolves away and leaves a hollow mould. If other material later fills that hollow, you get a cast, which is a different kind of fossil again.
More than bones
A footprint can survive if sediment covers the track before anything destroys it. Fossil eggs and nests reveal other sides of an animal’s life.
Maiasaura fossils include nests and young animals. In Archaeopteryx fossils you can still see the impressions of feathers. A single bone could never tell you that much.
Why fossils are rare
Most remains rot or are destroyed before they can be preserved. Even when a fossil does form, it may be damaged later, or stay buried where nobody ever finds it.
That’s why every find is only one small piece of the much bigger story of ancient life.
Where fossils are found
Fossils usually turn up in sedimentary rocks such as limestone, sandstone and shale. The layers of rock help scientists work out when and how the remains were buried.
A museum is a good place to start learning what fossils look like. If you find something outdoors, show a grown-up and check the rules before you collect it.
Questions kids ask
- How does a dinosaur bone become a fossil?
- Sediment can bury the bone and protect it. Over time, minerals may fill the spaces inside it or replace some of its material.
- Is every old bone a fossil?
- No. A bone thrown away not long ago is just a bone. A fossil is evidence of ancient life, preserved over a vast stretch of time.
- Why are fossils so rare?
- Most remains rot or are destroyed. Only some are preserved, and fewer still are ever found.
- Are footprints fossils?
- Yes. They are called trace fossils, because they show what an animal did and are not part of its body.
- Can I find one myself?
- Sometimes, yes. Go looking with a grown-up, and follow the rules of the place you are visiting.
- Is a mosquito in amber a dinosaur fossil?
- No. It is a preserved insect. Amber forms from tree resin, and it preserves things differently from the minerals that fill a bone.
Remember this
- A fossil does not have to be a bone. It can be a trace, such as a footprint.
- Draw layers of sediment around buried remains to show one way a fossil can form.
- Compare the Maiasaura nests with the feather impressions of Archaeopteryx.