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:

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.

Some examples of natural geological formations that may be mistaken for fossils are concretions. These are rhythmic deposits of iron around a node/core that may be organic e.g. the root of a tree. Dendrites are another formation that can resemble fossils of leaves but are of a mineralogical origin.

Common perceptions of what a fossil is and what it should look like are often a long way from the reality. Often we have a 'it looks like something therefore it is' mindset, and it can sometimes be very difficult to convince someone that their valued specimen is not what it appears to be and that it may not be of great value. However, many of these mineral formations are just as interesting as fossils.

Bones that are dug up from the ground may not always be fossilised.

  • Examine the bone for mineral matter that has come from its surroundings - if it is fossilised it will be relatively heavy.
  • Look for calcium carbonate encrustations - these indicate the specimen has been buried in an environment where ground waters have been rich in calcium carbonate. It is more likely that this happened in the more distant past than in recent times, so it may be an indication that the bone is a fossil.

The Australian megafauna became extinct within last 20,000 years and were reasonably common in New South Wales. Their fossils are most easily identified by the size of long bones and by the teeth. Skulls and jaws with teeth should be referred to Australian Museum Palaeontology staff if they can't be identified as recent bones.

A comparison with the bones of domestic and native animals can be helpful. Marsupials have distinctive premolars that are virtually species-specific.

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