Fossils are our window into the past. They are records of life preserved in monuments of stone. Almost all living organisms can leave fossils, but usually only the hard parts of plants and animals fossilize. Soft internal organs, muscle, and skin rapidly decay and are rarely preserved, but the bones and shells of animals are good candidates for fossilization.
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.
When are 'fossils' not fossils?
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.
The old fossil in question.
At the given coordinates you will find a fossilised tree. This fossil was 'found' circa 1900 by white man after being told of it's presence by by Aboriginals. This fossil was found in a salt pan east of Lake Eyre and is estimated to be 1 million years old. Wood becomes petrified when it is buried under sediment and the surrounding ground water saturates its tissues. The wood and its pore spaces become filled with minerals, in this case silicates, and a rock-like fossil results. Petrified remains of trees from the Cretaceous period (135–65 million years ago) and the Tertiary period (65–2 million years ago) are common in Central Australia and are also found further south at places such as Gawler and the Onkaparinga.
To successfully log this cache please email me the answers to following questions:
- Look at the fossilised tree. Describe the shape, colour and texture of the petrified tree at the coordinates. Describe the features of the original tree that can still be seen today.
- Look at the wall behind the fossilised tree. Can you find fossils in the limestone wall? What is fossilised here? Using the information provided above describing the formation of fossils how do you think these fossils were formed. Explain your reasoning.
Feel free to post your pictures of your visit in your logs, but, PLEASE, no photos that will give away the answers to the tasks.
Sources:
- The Australian Museum: www.australianmuseum.net.au
- The South Australian Museum: www.samuseum.sa.gov.au