Fossils are evidence
of life from the distant past. This means that the bones of a chicken you ate
last week are not fossils (not old enough), but the bones of Dromornis
(the Demon Duck of Doom) that lived 20,000 years ago are fossils.
Are all fossils
dinosaur bones?
Dinosaurs
were a fascinating and impressive lineage of animals. However, fossils are much
more than dinosaur bones. The most common fossils are the remains of tiny
marine creatures – shells and teeth.
There are
two general types of fossils: body fossils and trace fossils. A body fossil
preserves the remains of an organism. Bones, shells, leaf prints and insects in
amber are examples of body fossils. A trace fossil is an indirect sign of life
such as footprints, burrows, trails or coprolites (fossil poo).
These body
fossils include (from top left) petrified wood, trilobite, shark tooth, fish
skeleton and ammonite shell
This trace
fossil is a coprolite
How do
fossils form?
Most organisms
that die do not become fossils. In order to be fossilised special conditions
must be present. Most commonly, there must be quick burial in fine sediments with little decay. Over time the soft parts of an organism generally decompose,
and the sediments turn to rock. When this rock is exposed in the future, we
find the fossil.
Permineralisation
is the type of fossilisation that occurs when dissolved minerals seep into
spaces in the remains and then crystallise. Petrified wood is a common example
of this. Sometimes the minerals in the organism’s bone or shell may be replaced
by dissolved minerals. This is called replacement.
Moulds and
casts may be body or trace fossils. An impression (mould) of a trilobite is a
body fossil. A footprint is a trace fossil. The opalised fossils from Lightning
Ridge, New South Wales are casts of the original organisms.
Teeth are
very tough and are often preserved without replacement, like the shark tooth in
the photo above. These are actual remains. Insects trapped in amber are another
example of actual remains. The hard exoskeleton remains, but the soft interior has
decayed.
Carbonisation
occurs when organisms (usually leaves) are buried in sediments and a residue of
carbon is left behind. When a huge quantity of leaves are compressed, coal
forms. This is why coal is a fossil fuel. We can burn the carbon from plants
that lived millions of years ago.
Coal is
composed of carbonised leaf fossils
Webpage:
- Learn more about fossils
(University of California Museum of Paleontology) - Amazing
Australian amber fossils were described in April 2020
(ABC Science)
Explore
fossilisation for yourself
You can make
your own fossils and investigate different types of fossilisation:
Make moulds and casts with plasticine and
plaster of Paris
Create trackways in a sandpit
Do fieldwork to spot potential carbonisation and
make your own moulds and chocolate fossils by following instructions in the
video