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A Basic Introduction To Fossils
How They Are Formed

Fossils are found in rocks that were usually laid down under water (sedimentary rocks), such as shale and limestone. Some fossils, such as woolly mammoths from Siberia, are very well preserved (they still have meat that is just about edible), but in most cases only certain hard parts such as bone or shell remain, and these have been partially or totally replaced by minerals that have seeped in from the surrounding sediments.

Fossilization is thus quite a rare event since only a small proportion of the creatures that die (especially those that live on land) are likely to be enclosed in sediments before they are eaten or decay completely. Other organisms, such as worms and flowers, have few or no resistant hard parts and so are less likely to be found as fossils.

The diagram below gives an example of how a fossil forms.

How A Fossil Forms

1. An Ichthyosaur dies 150 million years ago and its skull falls to the sea-floor.

2. Layers of sediment are deposited and the skull is crushed by the weight of the overlying material. The sediment hardens into rock. Components of the bone dissolve away to be replaced by minerals seeping from the surrounding sediment.

3. Earth movements raise the rock above sea-level.

4. Erosion exposes the fossil on a beach, where it is found.


The earliest fossils are those of tiny microbes which are about 3400 million years old but fossils only start to become relatively common in rocks that are younger than 600 million years. The age of a rock, and hence of the fossils it contains, is deduced by measuring the rate of radioactive decay of certain chemicals in different rock beds. The most recent fossils are several thousand years old; remains that are younger than this have usually not been replaced by any minerals and generally belong to types of organisms that are still alive today.

The Geological Timescale on Page 1 shows the major divisions into which geologists have divided the history of the Earth and the main events that have taken place. Basically, geological time is divided into four main sections called Eras. These are further divided into the Periods shown in the table. Although the names seem daunting they are just a convenient way of referring to different parts of the Earth's history.

Most of the Earth's history, from its formation 4600 million years ago to 570 million years ago, belongs to the Pre-Cambrian Era. It was towards the end of this time that the first simple life appeared on this previously barren planet.

The succeeding Palaeozoic Era (570 - 232 million years ago) saw an abundance of life (and hence fossils). The seas were inhabited by creatures such as trilobites, and later by a variety of fishes. The first land life appeared roughly halfway through the Era and later there were huge coal forests inhabited by insects, amphibians, and finally reptiles.

The Mesozoic Era (232 - 65 million years ago) is appropriately called the 'Age of Reptiles'. This was the time when dinosaurs dominated the land, pterodactyls ruled the air, and a variety of reptiles inhabited the seas. The first mammals appeared near the start of the Era; birds and flowers roughly halfway through. In the seas trilobites were extinct and ammonites became dominant instead.

The end of the Mesozoic Era saw the extinction of the dinosaurs, pterodactyls and ammonites. Instead, mammals and birds became more important. Throughout the Caenozoic Era life on Earth grew increasingly similar to that of today.

Man finally appeared near the end of the Caenozoic, in the Quaternary Period.

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