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But fossils are not usually the undisturbed remains of creatures that have been buried. Many of the fossils that we see do not look very much like the plants or animals when they were alive. What we have to do is to learn to interpret the remains, and for this purpose we need to understand how fossilisation takes place.
Fossilisation
Living things are made mainly of water. This water bulks up the soft parts that make up most of an animal or plant while it is alive. Only the skeleton is hard. Skeletons come in two kinds: an outside covering, such as a shell (called an exoskeleton), or an inside skeleton such as is found in mammals and reptiles.
Most major groups of animals with external skeletons (invertebrates) have a skeleton or shell
that is made of calcium carbonate, that is called a calcareous skeleton. Some other groups have shells of calcium phosphate. Bones of animals with internal skeletons (vertebrates) are made of calcium phosphate.
When a living thing dies, the water leaves the body and the soft parts decay. The vast majority of living things decay and leave no trace. What we see preserved in rocks is a tiny fraction of the former life on earth.
With the soft parts decaying rapidly, it is far
more common for only the hard parts to be preserved. Because the skeleton is often held together by soft tissue, the decay of soft tissue tends to allow the skeleton to fall apart. This is why it is more common to find parts of a skeleton – a single tooth, a leg bone, or a single shell – rather than a whole skeleton.
When any living thing is buried, the hard parts are protected from the wear and tear of the weather, waves, and currents, and have a better chance of being preserved. However, as more sediment
builds up on top of the skeleton, it will be put
under greater and greater pressure, so that finally it may be squashed into a different shape.
(Below) Trilobites are invertebrates and so have an external skeleton or shell. It is this that has been fossilised. The photo shows a cast. Nothing remains of the original skeleton.
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(Below) This fossil shark’s tooth is life size. As one of the hardest parts of the animal it belonged to, it is
all that was left of a giant predator that would have been about 25m long. It is easier for us to interpret what the ancient shark would have looked like because sharks have changed relatively little in millions of years of evolution.