How are fossils preserved?
When a living organism dies, it falls either to the ground or to the sea floor. Here, if it is quickly covered and buried by sediment, it can be preserved in the form of a fossil. If there is no rapid burial or sedimentation, the dead organism can be scavenged or moved away, and no fossil can form. It is generally the hard parts of organisms that are preserved, e.g. their skeletons and shells, as these are stronger and more resistant. Therefore, it can be said that the fossil record is biased to organisms with hard parts. Organisms can also be preserved in environments where there is no air e.g. amber, oil sands/oil spills/ tar pits, ice, bogs, and they are essentially 'mummified'.
Types of preservation
Replacement -> the original hard part of the organism dissolves away and is fully or partially replaced by a different mineral. There are several ways this could happen: permineralisation (mineral-rich water percolates through the sediment and precipitates out the mineral in the empty spaces, internal detail of the fossil is often preserved), impregnation/ embedding (similar to permineralisation but the fossil is also surrounded) and mineralisation (the complete replacement of the original material by a new mineral).
Carbonisation/ Compression -> high pressure, from compression, expels volatiles and can cause a dark imprint of the organism to be left behind on the rock. This commonly occurs with leaves and ferns.
Exceptional preservation -> this is where the soft body parts of organisms which are not usually preserved are also preserved. This usually occurs in environments where there is no oxygen and the organism is unable to decay and disintegrate. This is the case for organisms that become trapped in amber, oil sands/oil spills/ tar pits or even glaciers or bogs. The two most famous examples of this type of preservation are: The Burgess Shale in Canada, where remains of the first shelled creatures from the 'Cambrian explosion of life' have been found, and The Solnhofen Limestone in Germany, where soft-bodied organisms, such as jellyfish, and the earliest bird, the Archaeopteryx – complete with feathers, have been found.