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A fossil skull of a Tyrannosaurus rexA fossil skull of a Tyrannosaurus rex Fossils are remains of once-living things preserved in rock. Most living things are eaten or die and their soft parts rot away leaving no trace. But sometimes hard body parts remain, like the shells, bones, teeth, horns and claws of animals and the bark, cones and seeds of plants. These are the parts most likely to form fossils. Trace fossils are not actual body parts but signs and traces of living things such as animal burrows, footprints and droppings.

A Jurassic ichthyosaur fossil A Jurassic ichthyosaur fossil
How a fossil formsHow a fossil forms

How fossils form

This sequence of illustrations shows how a dinosaur that perished many millions of years ago came to be preserved as a fossil which we can study today.  

When a creature—in this example, a dinosaur—dies under water (1) the hard parts, such as bones and teeth, may be buried under sediment particles such as sand grains on a beach, silt on a river bank or mud on a sea bed (2). Slowly the surrounding water dissolves away the remains and replaces them with rock minerals from the water.

Meanwhile the particles around them also turn into rock. If undisturbed, the remains keep their original shape but they are now solid rock. They are now fossils (3). The remains of animals and plants that lived at the same time become fossilized in the same rocks. So scientists can use fossils to work out what life-forms existed at different times in the past.

Fossils take millions of years to form. As a result of Earth movements caused by plate tectonics and the wearing-away forces of erosion, the rocks in which fossils form may one day be exposed at or near the surface (4).

Experts called palaeontologists search for fossils, dig them from the ground, study their shapes and structures, and compare them with similar body parts of living things today. From this evidence, it is possible for them to recreate what dinosaurs, mammoths and other animals and plants that lived millions of years ago looked like, and even gather information about how they lived.

Fossils form only in sedimentary rocks. They are destroyed if the rock is heated or squashed too much so they do not occur in igneous rocks or metamorphic rocks. Some sediments contain layer upon layer of fossils, like certain types of limestone.

The oldest fossils known are of blue-green bacteria (cyanobacteria). Dating from around 3500 million years old, they were discovered in Western Australia.

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