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A fossil is evidence of life preserved in a geologic context. Most fossils are old (the BGS gives typically more than 10,000 years), although there is no fixed age.

Preservation needs rapid burial, which prevents decomposition, weathering and scavenging, and favours durable hard parts. The chance that any one organism becomes a fossil is very small, and the record is biased: about 99% of fossils found are of marine animals, while insects, birds and upland organisms are rare.

Modes of preservation include permineralisation (minerals from groundwater fill the spaces in cells; the most common), replacement (organic material is entirely replaced and the remains literally turned to stone), moulds and casts, compressions, and preserved remains in amber or ice, the rarest form.

Experiment: Coin model of radioactive decay

1. Put 100 coins in the tray: each coin is an undecayed 'parent atom'. One shake = one time step.
2. Shake the tray and tip the coins out. Remove every coin showing heads: these have 'decayed' into daughter atoms. Count the coins left.
3. Repeat with the remaining coins until fewer than 3 are left, recording the count after each shake.
4. Plot parent coins left (y) against number of shakes (x). Add the ideal curve 100 × (1/2)ⁿ.
5. From your graph, find the 'half-life' (shakes to fall from 100 to 50, and from 50 to 25). Is it constant?
6. Pool the class results and compare the scatter of a single group with the class mean.

Shake n	Parent coins left	Ideal 100 × (1/2)ⁿ	Daughter coins (total removed)
0
1
2
3
4
5
6

Questions

1. Use your coin graph to find the half-life of your 'isotope'. Explain why your result scatters around the ideal curve.

2. A volcanic ash layer dated at 150 million years lies directly above a layer with ammonite fossils. What can you say about the age of the ammonites, and why?

3. A rock contains 12.5% of its original uranium-235 (half-life 704 million years). Calculate its age.

4. Draw a sequence of four rock layers containing two index fossils, and show how you would match them with a sequence in another place.

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Sources for every fact on this sheet: echoesof.science/school/fossils/sources/

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