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Gradual replacement by minarles
Things like bones teeth and shells which don't decay easily last a long time when buried they are eventually replaced by minerales as they decay forming a rock like substance shaped like the original
Surounding sediment turns to rock too but the fossil stays distinct
From casts and impressions
Organism is buried in soft material like clay (moulds to the shape) the clay later hardens around it and organism decays leaving cast
A burrow or roots can be preserved to make casts too
Foot prints can be pressed into these when soft leaving impression when harden
From preservation in places no decay can happen
1)in Amber and tar pits there is no oxygen or mosture so decay microobs can't survive
2)in glaciers too cold
3)peat bogs too acidic
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