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Molluscs
Molluscs (Mollusca), which means soft bodied,
is the name given to a wide range of classes of animal. They include the cephalopods (squids and ammonites), lamellibranchs (clams, bivalves) and the gastropods (snails).
Cephalopods
Cephalopods (Cephalopoda) are represented
by the modern squid and nautilus, but they
are important, geologically, for two ancestral versions, the belemnite and the ammonite. Ammonites were important from the Devonian to the Cretaceous periods, and belemnites from the Carboniferous to the Cretaceous periods.
These animals had a shell (internal in the case of the belemnite, or ancient squid), which
is divided into a number of chambers. Air in
the chambers allowed the animals to alter their buoyancy and so live at any depth they chose. They were able to move quickly through the water by squirting jets of water.
Belemnites
Belemnites were first found in the Carboniferous Period and were common in the Jurassic and Cretaceous periods. However, because ammonites are usually found in the same strata and ammonites are even easier to identify, belemnites have not been used as widely as might otherwise be the case. Belemnites are, however, used to date the Late Cretaceous because by this time most ammonites had become extinct.
Recognising belemnites
Belemnites are unusual in having their shell inside their bodies.
The shell often breaks up into
two pieces, with the more fragile guard being lost, and the more robust phragmacone surviving. The phragmacone contains chambers, separated by thin walls. In many cases the fossilisation process replaces the whole phragmacone with a single piece of silica. Then
the phragmacone looks like a stone bullet. Just occasionally, especially in limestone, the phragmacone survives intact, and then a section through it shows curved lines, which represent the chamber walls.
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