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What are fossils?
Fossils are the remains or traces of animals or plants preserved by natural
causes in the rocks of the Earth's crust. For example, if an animal dies, and
is buried rapidly in a muddy sediment, its hard parts are likely to be
preserved as a fossil. Fossils of marine animals are by far the most common
because most sediments are laid down on the sea floor.
How are fossils preserved?
When an animal dies its soft parts usually decay rapidly leaving only the shell
or skeleton. The soft parts are, therefore, only rarely found as fossils.
Corals, snails (Gastropods), lamp shells (Brachiopods), and similar have
a 'hard' shell that is often preserved unaltered. Sometimes the shell may
be entirely replaced by a mineral such as Pyrite (Iron Sulphide, or 'fool's
gold') or it may be dissolved from the rock by groundwater leaving only
the impressions of the inside (internal mould) and the outside (external
mould) of the shell.
Plants
and Graptolites are usually crushed so that all that remains is a carbonised
impression.
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