School โบ Fossils โบ Ages 10โ11
How fossils form
Body fossils and trace fossils, the conditions a living thing needs to become a fossil, moulds and casts, and why most fossils are of sea creatures. With a fair test to find the best 'fossil mud'.
Read
Fossils are the preserved remains, or traces, of plants and animals that were buried in sediment. They are typically found in sedimentary rocks.[3]
Body fossils are parts of the living thing itself: bones, teeth, shells, wood or leaves. Trace fossils are signs of what it did: footprints, burrows, nests and coprolites (fossil dung). Trace fossils tell us about behaviour that we could not learn any other way.[5][6]
To become a fossil, remains must be buried soon after death. Burial protects them from rotting, weathering and scavengers. Hard parts such as shells, bones, teeth and wood last much better than soft tissue. Fossils form more often where sediment is building up, such as in the sea and on river floodplains.[4]
In the most common kind of fossil, water carrying dissolved minerals seeps into a buried bone, tooth, shell or piece of wood and fills its tiny spaces. Over thousands to millions of years it turns to stone.[6][1]
Sometimes the buried shell or bone dissolves away and leaves a hollow in its shape: a mould. If minerals or sediment later fill the mould, they make a cast.[1][3]
A few fossils keep the original body: insects in amber up to 100 million years old, or the baby mammoth Lyuba, frozen in ice about 40,000 years ago.[6]
About 99% of the fossils found are of sea animals, which were quickly buried by sand or mud. Sea fossils are even found on Mount Everest, lifted up by the movement of the Earth's plates.[1]
Mary Anning (1799โ1847) of Lyme Regis in England found an ichthyosaur skeleton as a young girl, and later the first complete skeleton of a plesiosaur ever discovered.[2]
Try it: Fair test: which 'mud' makes the best fossil?
You need
- 3 small trays or pots
- Dry sand, damp sand and play dough or modelling clay
- The same shell for every test
- A heavy book (to press with the same force)
- A magnifying glass
- Optional, for the teacher: plaster of Paris, a plastic cup and a stirrer
Safety
Only the teacher mixes and pours plaster; avoid breathing the powder, and never put hands into setting plaster because it gets warm. Do not eat play dough. Wash hands afterwards.
What to do
- Fill the three pots with dry sand, damp sand and play dough, each smoothed flat.
- Plan a fair test: what will you keep the same? (Same shell, same pressing force using the book, same time.)
- Predict which material will keep the clearest print.
- Press the shell into each material under the book for 10 seconds, then lift it out carefully.
- Score each mould for detail with the magnifying glass: 0 = no shape, 1 = rough shape, 2 = clear shape, 3 = fine lines visible.
- Optional: the teacher pours plaster into the best mould. When it has set, lift it out: you have a cast.
Play: Body or trace?
Is it a body fossil (part of the living thing) or a trace fossil (a sign of what it did)?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6โ7 ยท Ages 15โ16