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Three small ammonite fossils, each about 1.5 cm across

A fossil of a trilobite which lived about 444 million years ago

Lower Proterozoic stromatolites from Bolivia, South America. These were produced by cyanobacteria. Polished vertical slice through rock

A mosquito and a fly trapped in amber

Fossil locust from the earliest Upper Cretaceous, ~95 million years ago, Santana Formation, Brazil

A fossil is the remains or trace of an ancient living thing.

Fossils of animals, plants, or protists occur in sedimentary rock.

In a typical fossil, the body form is retained, but the original molecules that made up the body have been replaced by some inorganic material, such as calcium carbonate (CaCO₃) or silica (SiO₂). The fossil feels like, and is, made of rock. It has been mineralised or petrified (literally, turned into rock).