Types of Fossils
Original Fossils
Original fossils formed when a part of the
organism is preserved, with its chemical
composition being about the same as it
was when it was living. An original fossil
could be a complete skeleton, bones,
tooth/teeth or a shell.
Bone is composed of minerals (such as
calcium carbonate) that are resistant to
decay and which scavengers find difficult to
eat. The flexibility of living bone comes
from proteins within. These proteins
normally quickly decay after death, leaving
behind the minerals as hard but brittle
bones.
Original fossils commonly include:
● sea creatures that have shells, such as
molluscs like scallops, mussels and
clams
● vertebrates because they had teeth
and a bony skeleton
Examples are the remains of dinosaurs
and of Australian megafauna such as 3m
tall giant kangaroos, sheet-sized echidnas,
diprotodon, 3 metre long wombats and
marsupial lions (a skull of which is shown
on the next page - 45 000 years old, found
in WA).
Replacement Fossils
A replacement fossil forms when a part of
the organism is chemically changed into
another mineral. This takes a long time to
happen so most of these fossils date back
to over 60 million years ago.
Replacement fossils commonly form when
the calcium carbonate found in shells and
bony skeleton is replaced by another
mineral such as silica, also called silicon
dioxide. Silica is like sand. Sometimes the
bone or shell even turns into opal, another
form of silica. This means that the bone or
shell is now a lump of solid silica or opal.
This is shown on the next page.
If the materials being replaced is wood, it’s
called being petrified.
Carbon Film Fossils
Carbon film fossils (carbon trace fossils)
occur when the dead body partially decays
and leaves a thin black deposit of carbon.
On the right you can see a carbon film
fossil of an ancient fern. Lines are left in
the shape of the organism that decayed
and often show finer details.
Indirect Fossils / Trace Fossils
These are fossils that aren’t a part of the
organism, but traces left by them like
footprints, burrows, faeces, etc.
A mould is an imprint left in rock showing
the outside of an organism. It’s a negative
image, so it has space, but no body.
Molluscs often leave these moulds, since
the shell is covered in sediment, turned it to
rock, then the original shell is dissolved.
This happens commonly deep in the
ocean.
Sometimes even internal moulds can
happen e.g. a snail shell could fill up with
mud, and turn into rock.
A cast forms when an organism in rock
decomposes and the space fills with soil
that turns into rock. We can make these by
pouring plaster into footprints - like in
forensics.
Questions
1. Complete the table below
Type of
Description
Example
Fossil
Original
Replacement
Carbon Film
Indirect/trace
2. How old are most replacement
fossils?
________________________________
3. If wood is replaced, it is said to be
________________ wood
4. How could an internal mould be
created?
________________________________
________________________________
5. True or False: Forensics uses some
principles that are applied in
palaentology.