A Fossil is a natural mould or imprint of a previously living organism ie a fish, plant, dinosaur, insect or any other form of life.
Fossilization usually occurs in the water or the sea – with land-dwelling animals being washed down to the sea or dying in swamps or rivers.
Invariably the soft tissue decomposes and leaves the bones behind which are covered in silt.
Over millions of years more and more silt covers the area and under its immense pressure the layers of slit turns to rock.
The bone that is trapped eventually dissolves in the ground water, leaving a cavity which in turn gets filled with minerals. This forms a natural mould of the bones.
Then over more millions of years the rock is uplifted to the surface of the earth and erosion exposes the fossil. This fossilization is 'with alteration' and is called casts and moulds.
The other form is 'without alteration'.
There are three types of this form: -
Permineralization - where minerals leach into the cells of the organism, replacing the original cell.
A good example of this form of fossilisation is petrified wood.
There is another type of fossil known as Trace fossils. These are evidence that an animal was there. Types of traces fossils are footprints, tracks, trails and burrows.
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