How fossils form
Describe how remains or traces of ancient living things can be preserved as fossils, including in rock formed from layers of mud or sand
What a learner can do afterwards
- Describe one fossil-forming process: an animal dies, its remains are buried in mud or sand, layers build up, and the sediment becomes rock
- Explain that a fossil preserves remains or traces of ancient life, such as a shell or its shape in rock
- Give examples of common fossils (shells, bones, leaf imprints, footprints)
1 · Read
You have compared rocks before. Some rocks hold evidence of ancient life. A fossil preserves remains or traces of something that lived long ago. A shell can be one of those remains. A footprint is a trace left by an animal. Most dead plants and animals decay without becoming fossils.
Here is one way a shell fossil can form. An animal dies and leaves its shell. Mud or sand covers it. More layers build up. Over a long time, the layers are pressed together and minerals can join the grains into rock. The shell, or a mark of its shape, may be preserved.
Burial can protect remains from being eaten or washed away. A buried leaf may leave a print as layers become rock. Many fossils are millions of years old, but their formation times vary.
Body fossils preserve body parts or their shapes: bones, shells and leaf prints. Trace fossils record activity, such as footprints. An animal can walk away after leaving a footprint. Both kinds preserve ancient-life evidence. Sunlight alone cannot make an ordinary rock a fossil.
Fossils preserve remains or traces of ancient life. Burial under mud or sand is one route. A footprint can be left by a living animal.
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More to read
Where a fossil search begins
A fossil search begins with rocks. Many dinosaur fossils occur in sedimentary rocks, formed from layers of material. The rocks also need to be from a time when the animal lived.
Researchers use maps to locate promising layers. They look for fossil pieces exposed at the surface. Finding a fragment is a clue, not a promise that a whole skeleton lies underneath.
Keeping the clues around a fossil
Researchers record where a fossil lies before moving it. Its position and the surrounding rock help them understand the find.
Trained collectors remove rock carefully. Fragile fossils may need a protective plaster covering for transport. A find may be one bone or several pieces. Careful records still matter when much of the animal is missing.
Tracks from a living animal
An animal crosses soft mud and leaves footprints. It walks away. Later, more material covers the marks.
If those marks survive as the layers become rock, they can become trace fossils. A trace records something an animal did. Unlike a bone, a footprint is not part of the animal's body. The animal did not have to die to leave it.
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Where this leads
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