Definition · Plain-language
Fossils
Fossils are the preserved remains, impressions or traces of living things from the distant past, usually found in sedimentary rock.
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Remains and traces of ancient life
A fossil is any preserved evidence of past life. The word covers an organism’s actual remains — bones, teeth, shells, wood — and the marks it left behind, such as footprints, burrows or impressions. To count as a fossil, the evidence is usually very old, conventionally thousands of years or more. Fossils are found overwhelmingly in sedimentary rock, because the gentle, layered way that rock forms can bury and protect remains. The heat and pressure that create igneous and metamorphic rock tend to destroy delicate biological structures.
How a fossil forms
Most fossils form through a sequence that begins with rapid burial. When an organism dies and is quickly covered by sediment — in mud, sand or volcanic ash — it is shielded from scavengers, oxygen and decay. Over long periods, more sediment piles on top and hardens into rock. Groundwater carrying dissolved minerals can seep into porous remains and replace them mineral by mineral, a process called permineralisation, turning bone or wood to stone while keeping its shape. Sometimes the original material dissolves away entirely, leaving a mould that may later fill to form a cast.
Body fossils and trace fossils
Palaeontologists divide fossils into two broad kinds. Body fossils preserve part of the organism itself — a bone, a shell, a leaf or, rarely, soft tissue — telling us what ancient life looked like. Trace fossils, also called ichnofossils, preserve evidence of behaviour rather than the body: footprints, trackways, burrows, nests and even fossilised droppings. Together they form the fossil record, the layered archive of life through time. Because deeper rock layers are generally older, the fossil record reveals how life has changed across hundreds of millions of years, providing key evidence for evolution.
Key facts
At a glance
- Definition: preserved remains or traces of ancient living things
- Found in: mainly sedimentary rock
- Forms when: remains are buried in sediment and mineralise over long ages
- Two kinds: body fossils (the organism) and trace fossils (its activity)
- Studied by: palaeontologists
- Why it matters: the main evidence for the history of life and evolution
Common misconceptions
What people often get wrong
Often heard: A fossil is always the original bone or shell of the animal.
Actually: Often the original material has been replaced by minerals, turned to stone while keeping its shape. Some fossils are only impressions, moulds or casts, and trace fossils preserve activity rather than the body at all.
Often heard: Fossils can be found equally in any type of rock.
Actually: Almost all fossils occur in sedimentary rock. The intense heat and pressure that form igneous and metamorphic rock usually destroy the remains before they can be preserved.
Often heard: Anything dug out of the ground that is old is a fossil.
Actually: A fossil is specifically the preserved remains or traces of once-living organisms. Ordinary rocks, minerals and human-made artefacts are not fossils, however old they may be.
Going deeper
