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What are they and
what are they good for?
Cross-section through the trunk of a petrified
tree fern (Palaeosmunda) from the Springsure
area. Late Permian (260 million years old).
Ammonite (Toxoceratoidas taylori) from the
Walsh River. Cretaceous (70 million years old).
Fern leaves (Cladophlebis) from Mutdapilli.
Jurassic (170 million years old).
The word “Fossil” is derived from the Latin word fossus,
meaning “to dig” and in the modern usage it is applied
to any remains, trace or imprint of a plant or animal that
has been preserved in the Earth’s crust since some past
geologic or prehistoric time.
Many fossils are beautiful, spectacular, or, in the case of some
fossils such as dinosaur skeletons, awe inspiring, but more
importantly fossils give scientists useful information. The most
important role that fossils play with scientists is that they are used to
determine the age of the rocks in which they occur. They
also provide information such as the environment they were
deposited in.
Mankind has had a long association with fossils. The Chinese
often considered fossilised mammal bones (including the
bones of Homo erectus, one of mankind’s ancestors) as “dragon
bones” and used them in “herbal remedies”. The Greek scholar
Aristotle (more than 300 years BCE) wrote about fossil shells
and how some of them resembled modern shells on the beach.
Leonardo da Vinci (1452-1519) also wrote about fossils and how
he interpreted them to be the remains of once living organisms.
Fossils provide other forms of direct knowledge about our planet,
for example, physicists conclude that the rotation of our planet
is slowing down. Fossil groups such as corals and some kinds of
shellfish show daily and seasonal growths within their skeletons,
thereby allowing us to count the number of days in a year in the
past. Studies show that the earth’s year, 280 millions of years ago,
was about 390 days long, and 400 million years ago it was 400
days long, thus confirming the work of physicists.
The oldest fossiliferous evidence for life on our planet is mounds,
called stromatolites, built by algae 3.47 billion years ago in
Western Australia. For most of earth’s history, bacteria have been
the dominant life form. There is some evidence for worm-like
creatures going back about 1 billion years, but it is not till the
beginning of the Cambrian (542 Million years ago) that life began
to evolve rapidly. On the bottom of the inside of this pamphlet is
a time line of the last 542 million years pointing out the highlights
for life over this time span. On the timeline below Ma is an
abbrevation for “million years ago”.
William Smith (1769-1839) was an English engineer involved in
building numerous canals. In the course of his work he observed
that rocks of different ages contained different assemblages of
fossils and that the order of these assemblages was regular and
consistent. He determined that rocks from distant locations
could be correlated as the same age as each other on the basis
of their fossil assemblages. He referred to his observations as
the principles of faunal succession, however, working before
Charles Darwin developed his theory of evolution, William
Smith had no way of explaining how the fossil faunas changed
from one assemblage to the next. This work started by Smith
was continued by other geologists and ultimately the last
542 million years of our planet’s history were broken up into
different Periods on the basis of their fossil faunas.
Common life
Brachiopods, trilobites,
molluscs, sponges
Brachiopods, trilobites,
molluscs and
graptolites
Brachipods, coral (rugose Brachiopods, molluscs,
echinoderms, coral reefs.
& tabulate), graptolites,
molluscs, echinoderms
First Appearance
Shelly fossils &
vertebrates
Corals, bryozoans, fish
and land plants
Land animals (centipedes
and arachnids), land
vascular plants
Special Events
Animals evolve
skeletons. Towards the
middle of the Cambrian,
the first arthropods walk
on land
Plants colonise the land
near the end of the
Ordovician
Time Period
CAMBRIAN
ORDOVICIAN
542Ma
485Ma
SILURIAN
443Ma
Brachiopods, corals,
molluscs, echinoderms,
lycopod forests, tree ferns
amphibians
Land vertebrates
(amphibians), tree sized
lycopod plants, insects
Winged insects & conifers
First vertebrates walk
on the land
Trilobites become rare
DEVONIAN
CARBONIFEROUS
419Ma
359Ma
Tabulate Coral (Halysites) from the Charters
Towers area. Early Silurian (440 Million years old).
Gastropod (Keenia) from Cracow.
Early Permian (295 million years old).
The chances of any individual animal being fossilised is very unlikely
since most parts of an organism decompose rapidly after death.
Fossilisation often requires unusual circumstances such as rapid burial
or transportion to an anoxic environment (very low in oxygen).
Organisms with durable skeletons have a better chance of being
fossilised, e.g. corals are very common in the fossil record, but jellyfish
and sea-anemones are extremely rare.
The nature of the organism, the environment of fossilisation, and the
geological history of the rock that the fossil is contained in, all have an
effect on the survival of a fossil.
1) Unaltered Hard Parts – Under exceptional, near-perfect preservation
circumstances, body parts of certain animals can be preserved in their
original state. These exceptional environments are relatively rare, but
well known. When sealed in sedimentary rocks impervious to ground
water, dinosaur bones 100 million years old can still be made up of the
original bone material. Fossil shells can be preserved with their
original internal mother-of-pearl layer and external colour patterns
intact even after 500 million years. Trilobites over 500 million years old
have been found with chitin from their original skeletons preserved.
2) Recrystallisation – Most shelly fossils such as snail, clams, corals
and ammonites produce a skeleton made out of the calcium
carbonate mineral known as aragonite. In most environments of
fossilisation the aragonite is unstable and recrystallises to the calcium
carbonate mineral called calcite. In most cases the recrystallisation is
slow and at such a fine scale that the microscopic details of the
skeleton are usually preserved.
3) Replaced by other minerals (Permineralisation/Petrification) –
After an organism is buried, ground water can precipitate minerals into
the cavities in the organism that are filled with gas or liquid.
This replacement can occur on a very small scale such as single cells to
Trilobite (Xystridura) from Mount Isa.
Middle Cambrian (520 million years old).
produce extremely well preserved fossils. These sorts of fossils are best
represented by petrified wood where the woody tissue is replaced by
silica and original features such as wood-grain and growth rings are
preserved. Most fossilised bones are permineralised.
4) Carbonisation – Carbonisation is common with land plants and
animals that contain a large amount of organic matter such as leaves
or fish. The pressure of the overlying rock flattens the organism and
the removal of oxygen ultimately leads to the organism being
composed of almost pure carbon. This film of carbon can be preserved
as a jet black imprint of a fish or leaf. Alternatively the carbon can be
removed by groundwater to leave a detailed impression of the
organism. One of the best examples of carbonisation is coal which
consists of plant matter that has been compressed and predominately
turned into carbon.
5) Casts and Moulds – Sometimes, after the sediment surrounding
the organism has hardened to rock, the remains can be dissolved away
leaving an organism-shaped cavity in the rock. This cavity is called a
mould. Later this cavity can be filled with a new mineral but will retain
the shape of the original organism and this is known as a cast. A well
known type of cast fossils are opalised shells and bones, but they can
also be replaced by pyrite or calcite. With this type of replacement, all
microscopic structure of the original organism is lost.
6) Trace Fossils – Trace fossils are structures such as burrows,
footprints, feeding marks, nests, eggshells, and droppings that were
created by organisms and then preserved in sedimentary rock.
7) Pseudofossils – Certain inorganic structures can be mistaken
for fossils. The most common pseudofossils are delicate, branching
mineral growths known as dendrites. These are precipitated by water
into cracks and bedding planes and are often confused with plant
fossils. Nodules that form in sedimentary rocks often have unusual
growth shapes that can be mistaken as fossils.
Molluscs, brachiopods,
echinoderms, corals,
insects, Glossopteris
flora in Gondwana
Molluscs, crustaceans,
insects, dinosaurs,
conifers and cycads
Dichroidium flora spreads
throughout Australia
Molluscs, crustaceans,
ammonites, dinosaurs,
insects, conifers and
cycads
Bivalves, gastropods,
ammonites, crustaceans,
dinosaurs and insects
Bivalves, gastropods,
crustrations, birds,
mammals, flowering
plants and insects
Bivalves, gastropods,
crustaceans, birds,
mammals, humans,
grasses, flowering plants
and insects
Mammal-like reptiles,
beetles, cycads
archosaurs (ancestors
of the dinosaurs)
Dinosaurs, large marine
reptiles, pterosaurs,
mammals, modern
(scleractinian) corals
Birds
Snakes & monotremes.
Flowering plants
originate and diversify,
early marsupials
Whales, seals, bats,
rodents, horses,
monkeys, apes,
hominids and grasses
Modern humans
Extinction of rugose
and tabulate corals
and trilobites at end
of Permian
Ammonites diversify
rapidly
Diversification of
dinosaurs
Extinction of dinosaurs,
ammonites, large marine
reptiles at end of
Cretaceous. Birds
diversify
Dramatic diversification
of mammals and birds
Extinction of most
of the mammalian and
marsupial megafaunas
PERMIAN
TRIASSIC
JURASSIC
CRETACEOUS
TERTIARY
QUATERNARY
299Ma
253Ma
201Ma
145Ma
66Ma 2.6Ma
0Ma
The Australian Institute of Geoscientists (AIG)
represents professional geoscientists on a range of
public issues, promotes geological education
and development of members and students,
and provides venues for developing and maintaining
standards of professional practice.
Visit www.aig.org.au/branches/queensland
Star Fish from Melbourne.
Late Silurian (420 million years old).
The Geological Society of Australia (GSA)
is a non-profit organisation which disseminates
geological knowledge to professional geologists and
government, improving public awareness of geological issues,
and promoting better stewardship of our natural
geological resources and environment while helping meet
community health and economic needs.
Visit www.gsa.org.au and www.qld.gsa.org.au for more information.
Crinoid (Jimbacrinus) from the Gascoyne
district of Western Australia.
Early Permian (285 million years old).
Links to sites with information on
fossils in Queensland
Brachiopod from the Charters Towers area.
Middle Devonian (385 million years old).
Queensland Museum
http://www.qm.qld.gov.au/
Kronosaurus Korner
http://www.kronosauruskorner.com.au/
Australian age of Dinosaurs Museum
http://australianageofdinosaurs.com/museum-hub.php
Australia’s Dinosaur Trail
http://www.australiasdinosaurtrail.com/
Riversleigh Fossil Centre
http://www.outbackatisa.com.au/Attractions/
Riversleigh-Fossil-Centre.aspx
Dinosaur (Muttaburrasaurus) at the Queensland
Museum. Trace fossils of its footprints are
displayed on the wall behind the skeleton.
Cretaceous (130 million years old).
Branches of Tabulate corals (Thamnopora) and
Rugose corals (Amaraphyllum) from Charters
Towers area. Middle Devonian (385 million years old).