What are fossils?

The word ‘fossil’ comes from the Latin word fossus, which means ‘dug up’. This refers to the fact that fossils are the remains of past life preserved in rock, soil or amber. Generally, the remains were once the hard parts of an organism, such as bones and shell although, under exceptional circumstances, soft tissues have also fossilised. There are different types of fossils because remains can be preserved in a variety of ways.

Trace fossils

Trace fossils are the preserved evidence of an animal’s activity or behaviour, rather than the remains of the animal itself. Examples of trace fossils that have been found include:

• trilobite tracks
• marine invertebrate burrows
• ichthyosaur coprolites (fossilised poo)

Fossils with some organic material preserved

Animal and plant remains can undergo a variety of physical and chemical changes during fossilisation. This results in fossils showing varying styles and degrees of organic preservation:

• minimal decay with only some loss of soft tissue
• preservation of a skeleton with minimal change
• removal of all organic material except carbon, which remains as a film in the rock

Mineralised fossils

These fossils are formed when organic matter in remains is gradually replaced with minerals, ultimately turning the remains to ‘rock’. Types of replacement include:

• Replacement of organic material in bone with minerals
• Replacement of shell with pyrite
• Replacement of shell with opal
• Replacement of wood with silica – petrified wood

Impression fossils

Fossils can also form when the remains decay completely but leave an impression in the sediment. Impressions can be of the external shape or internal space (which may fill with other minerals or sediment).Examples include internal impressions of trilobites or ammonites and external impressions of armoured fishes or tree bark.