Fossils (from Latin fossus, literally "having been dug up") are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. Using radiometric dating techniques, geologists have determined most fossils to be several thousands to several billions of years old. Fossils vary in size from microscopic, such as single cells, to gigantic, such as dinosaurs.

Fossilization is actually a rare occurrence because most components of formerly-living things tend to decompose relatively quickly following death. In order for an organism to be fossilized, the remains normally need to be covered by sediment as soon as possible. However there are exceptions to this, such as if an organism becomes frozen, desiccated, or comes to rest in an anoxic (oxygen-free) environment such as at the bottom of a lake (e.g. Fish Fossil). There are several different types of fossils and fossilization processes.

Ammonite - 135 -395 Million Years old from Madagascar. Ammonites were marine animals, similar to squid or octopus but with a coiled external shell which helped to float and protect the creature inside. This Shell is usually the only part of the ammonite to be preserved as a fossil.

Knightia Alta - 55 Million Years Old from Wyoming, USA.
These beautiful preserved fish are found in the famous Green River Shales of Wyoming, USA. A relative of the modern day herring, these fish were probably at the bottom of the food chain, eating algae and small single cell animals.

Trilobites are very well-known, and possibly the second-most famous fossil group after the dinosaurs. The last of the trilobites disappeared in the mass extinction at the end of the Permian period about 250 million years ago.

Goniatites were free-swimming marine animals with eyes and tentacles which inhabited a coiled external shell. They have distinctive 'zigzag' pattern on the shell, although the shape of the shell suggests they were poor swimmers.

Orthoceras were amoung the eariest recognisable animals, ancestors to the modern day squid buy with an external shell. By filling the chambers of this shell with water and then squirting it out, they could propel themselves through the sea.

Petrified wood is fossilised wood and is found in a great variety of shapes, forms and colours. The organic material is replaced with minerals and crystals - largely quartz - and the different minerals present give the fossil a variety of colours. For example, cobalt produces greens and blues, iron oxides look red or brown and carbon creates black markings.

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