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Fossils are the remains or traces of prehistoric plants and
animals preserved in rocks. They usually comprise the hard
parts of the animal or plant, or structures resistant to decay.
So any organism which has shells, bones, teeth or wood is
likely to be preserved as a fossil, examples include molluscs,
mammals and trees. Less common are fossilised hard parts or
traces of soft parts of animals or plants, hence worms and
have a much smaller chance of being preserved. Footprints
and burrows of animals, along with plant roots can be preserved
to become fossils and these are called
The study of fossils is called palaeontology and early cultures
such as the Greeks were studying fossils in the fourth and fifth
centuries BC. However, the scientific study of fossils was really
begun by Carl
Linnaeus and George
Cuvier in the late eighteenth and early nineteenth centuries.
fossils are formed
Animals or plants that become fossilised are buried by sediments,
for example sharks fall to the ocean floor after they have died
and are then covered by sediment. As the sediments pile up on the
remains of the animal or plant, the pressure increases, some water
may be expelled and chemical changes occur to produce sedimentary
rocks such as
In harder rocks such as limestone, fossils are more likely to retain
their original appearance, while softer rocks may compress or flatten
Fossil examples, that some say are the world's most significant
are those in the Burgess Shale, Canada. The Burgess
Shale fossils are special because of their great age, and their
There are a variety of ways to form fossils:
- complete preservation in a relatively undamaged state of hard
and soft body parts;
- skeletons and shells
- these are the most common fossilised hard parts; they may loose
their colour but are unchanged in their chemical composition;
- petrification and replacement
- impregnation of fossils by secondary minerals which usually
leads to an increase of weight and hardness of the fossil;
- usually formed from woody and chitinous
material, which loses its oxygen and nitrogen through
decomposition by anaerobic bacteria;
impressions and traces - impressions left
by dead animals that decayed away or traces left by moving
- casts and moulds
- where the rock containing the fossils has hardened and the original
fossil has been dissolved away, leaving a hole which can often
be filled by other sediment.
Once they are imbedded, fossils experience the same geological history as the rocks they are incarcerated in. Rocks may be squeezed and distorted with fossils contained inside. Heat and pressure often puts fossils into a sorry state by the time they are found. The resulting fossils are often found as fragments and piecing them together is just like doing a jigsaw. Some species have been identified by only one fragment of their skeleton. Some fossils have travelled great distances through the forces of plate tectonics and mountain building. Ammonites for example, which have died and fallen down to the ocean floor, can be found in rocks at an altitude of over 5000 metres in the Himalaya Mountains.
Types of fossils
Fossils can be grouped into macrofossils, microfossils and trace
fossils. Macrofossils are fossils that can be easily seen with the
unaided eye. Animal macrofossils are predominantly shells, bones,
skeletons, calcified skeletons, fish scales, sometimes eggs or impressions
of soft parts. Plant macrofossils are mostly woody branches, trunks,
stumps, roots, leaves, seeds, cones or fruit.
Microfossils are fossils that can be only seen in detail with a
microscope. They are generally smaller than 1mm. Animal microfossils
include small fish teeth, fish earbones, worm jaws, and spines of
sea urchins, and internal or external skeletons of small animals
like the waterflea (Daphnia). Most common however are the
hard parts of marine
(single-celled organisms), called
Their minute skeletons sink to the ocean floor when they die and
are well preserved as fossils, contributing hundreds of tonnes of
sediment to the modern sea floor each year. Plant microfossils include
pollen, spores, marine algae and are, together with the protozoa,
the most abundant fossils to be found in rocks.
Trace fossils are left by moving animals, and may include the back filled
burrows by active sediment eaters such as worms or shelter burrows used for
retreat by crabs or shrimps. They also record the impression of organisms which
stopped to rest on the sediment.
For pictures of fossils found in New Zealand check out our fossil
record and evolution
Theoretically it should be possible to collect fossils of plants,
animals and other organisms throughout the
from the oldest rocks through to the youngest to read and explain
the past. Anyone who set out to do that would be disappointed due
to the gaps in the sedimentary rocks which contain the fossils.
Often fossils don't appear where they are "supposed to" simply because
they were not preserved. Occasionally fossils have been found in
unexpected places and this requires a re-think of some aspects of
the record. Unfortunately it is not possible to find an entirely
unbroken succession of fossil record that would span all of Earth's
history, although some places like the Grand
Canyon in the USA have a vast period of time preserved in successive
layers. New Zealand also has several places where rock successions
can be studied in sea cliffs or river beds or mountain bluffs. By
comparing fossil records from different locations, it is possible
to see a succession of fossils, which provide evidence for evolution
and form the basis of the geological time scale.
One of the biggest factors that affect the fossil record is the
way new species
develop, also called speciation.
Often, a population that gives rise to a new species exists at the
fringe of a larger parent population; it is isolated from the main
group and exposed to different environmental conditions and selection
pressures. Because low numbers of individuals are involved, and
because fossilisation is a rare event anyway, it is not always easy
or possible to find fossils of that new species in that new environment,
so that the fossil record of a particular lineage may at times resemble
a mixed up jigsaw.
There is little doubt that all living things on Earth share one
common ancestor. This insight was first documented by Charles
Darwin in 1859 in "The Origin of Species", and the
discovery of new evidence of evolution has repeatedly confirmed
his conclusion. One major indicator of a single common ancestor
is the fact that all life forms share common mechanisms for transmitting
inherited information (the
and using this information to control cellular processes (RNA
genetic instructions carried in the DNA are translated into proteins).
Two major lines of evidence available to Darwin and his contemporaries
were transitional fossils - the so-called "missing links" - and
the presence of shared derived characteristics, or
features (where "derived" means altered from some ancestral form).
Darwin himself recognised (when he was preparing the "Origin") that
the absence of fossils that were transitional between two other
species was a major problem for his theory of evolution. Some people
may argue that if all of life is related through a single huge family
tree extending from the present day back hundred of millions of
years to a single point of origin, we should find fossils that are
midway between established groups - so called transitional fossils.
They claim that none exist, and that this is conclusive proof that
Darwin, and generations of biologists, were wrong.
However, the first transitional fossil, or missing link, was found
in 1861, shortly after Darwin's Origin of Species was published.
The first specimen of Archaeopteryx was discovered in a
limestone quarry in southern Germany. Scientists immediately noticed
that Archaeopteryx was an intermediate form with both avian
and reptilian characteristics: it had feathers and wings, but also
a long bony tail, fingers with claws on the forelimbs, and teeth
in a heavy jaw. Seven more skeletons of Archaeopteryx have
been found to confirm those first findings. Other species of early
bird, discovered in Spain and China, are 30-40 million years younger
than Archaeopteryx, suggesting that it was not the" first
Often however, evolutionary changes are best preserved in rather
inconspicuous ancient life forms.
for example, are small planktic animals that show a very good record
of continuous changes due to a changing environment.
A number of sites provide excellent information on fossils and transitional forms:
The "Talk.Origins" site provides excellent information and discussion
On the Discovery of Global Warming website hosted by the Centre
for History of Physics of the American Institutes of Physics you
can find pictures showing the gradual change of the foraminifera
a good example of a well documented transition between species.
Other scientists have set up websites with collections of interesting
information. For example Andrew MacRae from the Dept of Geology
and Geophysics, The University of Calgary, Canada set up a site
which includes this information about trace
Zealand GNS website
has coverage of fossil research by NZ scientists.
Geological Institute has a great website about evolution and the fossil record.
For learning activities and teachers resources check out these
two sites. The Berkeley
learning activities about fossils macro and microfossils and is
a resource prepared for teachers.
Also great online teacher resources (ready-to-go-classes) about transitional
fossils can be found on the action.bioscience
The Royal Ontario Museum has a fossil
cyber-display which was
created by Irene Chalmers of the Discovery Centre with help from
Janet Waddington of the Department of Paleobiology. This site provides
many nice images.
A UK orientated website called KidsArk
has information about
fossils and fossil collecting. This website was constructed by the
Wilkinson family and is very kids safe.
Conway, Morris S. (1999). The Crucible of Creation: The Burgess
Shale and the Rise of Animals. Oxford University Press.
Erickson, J. (1992). An introduction to fossils and minerals:
seeking clues to the Earth's past. Facts on File, New York.
Fortey, R. (2002). Fossils: the key to the past. London:
The Natural History Museum
Hayward, B. (1989). Trilobites, Dinosaurs and Moa bones.
Auckland: The Bush Press
Monks, N. and Palmer, P. (2002). Ammonites. London: The
Natural History Museum