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(Right and below) A remarkable set of coincidences give rise to fossils and their discovery, as shown in the following sequence.
(a) Organisms die all the time. The cause may be old age, or they may have been killed by other organisms. Occasionally freak events, such as a storm or flood, may also cause large numbers to die at the same time and in the same place.
(b) Once dead, organisms normally decompose very quickly.
However, under special circumstances, decomposition can be slowed down. For example, a living thing might be buried rapidly in a landslide or flood, or it might come to rest on the bed of a lake in which there is little oxygen.
Under these special circumstances the organism can be preserved. Over time it can be buried by more deposits.
(c) Even the hard parts of a buried organism may decay, leaving only
an imprint in the surrounding rock. Alternatively, the hard parts may be replaced by other minerals, a process known as mineralisation. If the
rocks in which the fossils are buried are affected by heat or pressure, the minerals may change yet again, or the fossil may be destroyed.
(d) If the rocks come to the surface, they will be eroded. Fossils will be exposed for a while, because they are often harder than the layers of rocks that buried them. But, once exposed, it is only a chance encounter that results in the fossil being discovered before weathering or erosion destroys it.
Dragonfly
Tree
Brachiopods
Fish
Soft parts decay, leaving hard parts of organism.
The organisms are buried. Over time, the remains may be replaced by minerals.
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