WHAT IS A FOSSIL?

Fossils give us a hint of strange creatures that walked the earth before man existed. There is an universal appeal about holding something that is millions of years old in your hand. 

Fossilisation is a term for a number of processes that preserve organic remains. Fossils can include plant and animal remains, impressions and moulds.  Ancient fossil groups include dinosaurs, ammonites and trilobites.  

They can be formed in several ways. Dead animals and plants can be preserved in amber, peat bogs, tar pits, or in ice. Casts or impressions, such as foot prints, can be covered by layers of sediments which eventually become rock and so preserve the casts. Hard body parts such as bones, shells and leaves, can be covered by layers of sediments and over time the parts are gradually replaced by minerals.

Most organisms decay without leaving a fossil record but sometimes conditions are favourable for preservation of plant and animal material, and this is when fossils are created. 

There is no defined scientific date for a fossil, although it is broadly accepted to include items over ten thousand years old.  The oldest fossil recordings date back 3.5 billion years. 

Fossilisation can occur in several ways but the key to successful fossilisation is quick burial, low oxygen levels and minimal disturbance.  A rapid burial of sediment can entomb a specimen and protect remains from damage by heat or compaction. It also prevents disturbance of the body by predators. Low oxygen levels slow the decay of the specimen enabling better preservation.  

Over time more sediment is deposited on the specimen. The weight of this sediment compacts the sediment grains together, pushing out any water.  The soft sediment slowly turns to rock in a process called lithification. Minerals contained within the sediment slowly replace the minerals present within the skeleton, filling any voids left as the skeleton dissolves.  This leaves a re-mineralised copy of the original skeleton which is the fossil we see today.

Fossils are particularly preserved when sediments are deposited beneath water. It is often possible to trace their movement in the fossilised sediment. Even fossils preserved on land are more commonly preserved in wetlands, lakes, rivers and estuaries.  

Arguably, one of the most famous dinosaur fossils discovered is the feathered Archaceopteryx, which was found in 1861.  This feathered dinosaur lived in the late Jurassic period 150 million years ago and showed the link between dinosaurs and birds. Since than more than 20 species of dinosaurs with feathers have been discovered. 

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