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Reading activity 1: Animal fossils of
Gondwana
Read the following information, highlighting or underlining the answers to the following
questions:
1. What are fossils?
2. Where do fossils form?
3. Where are fossils found?
4. What types of things become fossils?
5. Who studies fossils?
6. Where is the best place to find fossils?
7. How are fossils studied?
The word fossil comes from the Latin word that means to ‘dig up’. A fossil is the preserved
remains or an impression of the remains (whole or part) of an organism. This organism
could be plant or animal. An organism that is fossilised doesn’t need to be an extinct
species. The organism is, of course, dead but the species it belongs to may still be alive.
Sometimes its ancestors or near relatives have been fossilised but its descendants and
‘cousins’ are living today.
Fossils are usually found in sedimentary rocks. Most sedimentary rocks are made up of
particles such as pebbles, sand and mud. These particles may be compressed and cemented
together to become rocks. This rock forming process usually happens at the bottom of lakes,
rivers or in the sea where the particles (sediment) have settled in layers. It can also happen
in wind-blown sand dunes. The layers of sediment can sometime contain the remains
of dead organisms. If conditions are right, the layers will consolidate into rocks, and the
preserved remains of the dead organisms will become fossils.
What are the preserved remains? Living things are composed of a mixture of compounds of
carbon as well as small amounts of other elements and inorganic compounds. Most carbon
compounds decompose over time and usually, very little of the original organism remains in
the rock, apart from its shape. As it is fossilised, the remains of the organism may change
from its original organic composition.
One way this change happens is by the action of ground water. As it seeps through layers of
porous sediments (just as in a kitchen sponge), the water dissolves minerals from the rock
particles. It then seeps into the organism and dissolves the organic remains, replacing them
with those minerals. In some cases minerals are added to organic material – a process called
permineralisation- so that both the original carbon-based material and new minerals are
present.
You may recognise that replacement of the original material has taken place if:
• The fossil is the same colour as the rock it is found in.
• The fossil is heavier than the original shell or bone would have been.
• The fossil looks different from the original organism.
Animal fossils of Gondwana 11
The inside structures of the organism may not be preserved. A fossilised tree trunk may
show no growth rings or cell structure. Sometimes cell structure and tree rings are preserved
if the mineral replacing the organic remains is silica, a glass-like mineral. Fossil wood from
Winton in Queensland and the South coast of NSW are examples. A petrified forest has been
discovered in the USA.
Hard parts of organisms such as shells, teeth, bones and leaves are most likely to be found
as fossils. Hair and faeces (called coprolites or fossil ‘poo’) may be fossilised. Eggs may be
fossilized, sometimes with the unhatched young animal still inside.
Very often imprints are all that remains of an organism. For example, feathers and soft
animals like jellyfish leave an imprint in the surface of the sediment layer they settle on.
Jellyfish have no hard parts to preserve so only an imprint is possible. New sediment covers
that layer filling in the hole left by the soft material that has long since rotted away. Because
they don’t have hard skeletons there is very little chance of the animals themselves being
preserved as a fossil. Other records of organisms include footprints, tracks or trails they have
left behind.
Most organisms that ever lived have not been fossilised so any fossil evidence found is
precious to science. Scientists who study fossils are call palaeontologists, pronounced – pailee-on-tol-o-gists. A palaeontologist studies the history of life on earth and all the things that
can happen to a living animal or a plant from the time it died until it’s fossils are discovered.
You can study to become a palaeontologist at university, although anyone with a real
interest can be involved with palaeontology. Australian amateur palaeontologists have made
some important discoveries during the last century.
A palaeontologist will select and search an area made of sediments or sedimentary rocks in
which they think fossils may have been preserved. When they find a small piece of the fossil
poking out they will then dig carefully to uncover the fossil, mapping where the fossil came
from and what it looked like in the ground. They will also look around the area very carefully
to see if there are any more fossils to be found.
Sediments and sedimentary rock are the best places to find fossils. River beds, beach areas
with cliffs, ‘badlands’ (eroded country in the drier areas of Australia) and road cuttings
are good places to look. Anywhere there are sedimentary rocks like sandstone shale and
mudstone is a good place to look for fossils.
When palaeontologists find fossils they have to try to identify what type of organism the
fossil came from. If the fossil looks like another fossil that has already been identified,
the palaeontologist will compare them to see if they are exactly the same. In these cases,
identifying the fossil is easy. If they are different, the palaeontologist will then have to
investigate all types of different organisms to decide what the fossils may be.
After they identify the fossil they also will need to investigate the types of rock in which
the fossil was found, how old the fossil is, how that organism became fossilised, and
many other puzzles. If the fossil turns out to be a new animal or plant not seen before, the
palaeontologist will give the fossil a name, often using Latin of Greek. Sometimes other
languages, such as one of the many Aboriginal languages are used as the basis for the name
of the new find.
12 Animal fossils of Gondwana