How Fossils Form

A message from deep time

A fossil is a kind of natural time capsule — the preserved remains or traces of a living thing from long ago, often millions of years old. Fossils are how we know dinosaurs walked the Earth, how we trace the story of evolution and natural selection, and how we read the history of life written in the rocks. But fossils are surprisingly rare, and the journey from living creature to museum specimen is a long and lucky one.

Step by step: how a body fossil forms

Most fossils form through these stages:

1. Death and rapid burial. An animal or plant dies and is quickly covered by sediment — mud, sand or silt — often at the bottom of a lake, river or sea. Speed matters: if the body lies exposed, scavengers and decay destroy it first.
2. Soft parts decay. Skin, muscle and other soft tissue usually rot away, leaving behind the hard parts: bones, teeth and shells.
3. Burial deepens. More layers pile on top. Over thousands of years, the sediment is compacted and cemented into sedimentary rock.
4. Permineralization. Mineral-rich water seeps into the tiny pores of the bone or shell and crystallises there. Slowly, the remains are turned to stone while keeping their shape — sometimes in stunning detail.
5. Uplift and exposure. Millions of years later, the rock is pushed upward by Earth movements and slowly worn away, until the fossil finally appears at the surface — where, if it is lucky, a person finds it before erosion destroys it.

Different kinds of fossils

Not every fossil is a turned-to-stone bone. The main types are:

• Body fossils: the actual remains — bones, teeth, shells, leaves — usually mineralised.
• Mould and cast fossils: the organism dissolves away, leaving a hollow mould; if minerals later fill that hollow, they form a solid cast shaped like the original.
• Trace fossils: records of behaviour rather than body parts, such as footprints, burrows, nests and even fossilised droppings (coprolites). These tell us how animals moved and lived.
• Preservation in amber or ice: insects can be trapped whole in tree resin that hardens into amber, and mammoths have been found frozen in permafrost with skin and hair intact.
• Petrified wood: tree trunks where minerals have replaced the wood cell by cell, preserving the grain in stone.

Why fossilisation is so rare

The vast majority of living things leave no fossil at all. To become a fossil, an organism usually needs:

• Hard parts like bone or shell — which is why soft animals such as jellyfish and worms rarely fossilise.
• Fast burial in the right kind of sediment.
• To avoid being eaten, weathered or destroyed for millions of years.

Because of this, the fossil record is incomplete, like a book with most of its pages missing. Yet even the pages we have are enough to reconstruct an astonishing story — explored further in the history of life in geological time.

Try it yourself: make a mould-and-cast fossil

1. Press a clean shell, leaf or plastic toy firmly into a flattened ball of modelling clay or play dough, then lift it out. You now have a mould.
2. Mix some plaster of Paris (or just use more of a different-coloured dough) and pour or press it into the mould.
3. Let it set fully, then gently separate the two pieces.
4. The plaster shape that comes out is your cast — a copy of the original object, just as a fossil cast copies an ancient animal.

Compare your mould and your cast: one is the hollow imprint, the other the solid filling. Real fossil hunters find both kinds in the rocks, and together they let scientists rebuild creatures no human ever saw alive.