Belemnites are an extinct group (there are actually several different
Orders of Belemnites) of cephalopod molluscs. They first appeared in
the Devonian period about 416 million years ago, were particularly
abundant during the
Mesozoic Era, especially
during the
Jurassic
and
Cretaceous
periods, but went extinct during the
Cretaceous-Tertiary mass extinction.
Belemnites are believed have
been similar to modern squid, and were related to cuttlefish. They
had ten arms of approximately equal length, which were apparently
equipped with hooks for grabbing prey, and also an ink sac.
They also had an internal chambered shell (known as the "guard" or
"rostrum") which was enclosed by
muscular tissue.
Fossils of Belemnites are
common, so much so that they are sometimes used as "index fossils"
(Fossils used to determine
the relative date of rocks), particularly in the case of
Cretaceous period
chalk formations).
However, although occasionally fossils
have been found showing soft-tissue,
normally only the shell of the animal is preserved, and this of course
represents a tiny fraction of the total animal. The smallest shells
are less than ½ inch (about 1 centimeter) in length, but the
largest found are around 18 inches (46 centimeters) in length - suggesting
the largest Belemnites were probably around 10 feet (3 meters) in size.
Today, a type of Belemnite, Belemnitella Americana is the State Fossil of
the US state of
Delaware.
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