Ammonites - General Information
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Fossil ammonites are the only remains of an an extinct group
of cephalopods. Cephalopods are fast moving predatory marine
invertebrates. Ammonites existed on earth for about 330 million
years. Living cephalopods include squid, octopi and nautiloids.
Ammonites were nektonic (free-swimming) creatures which lived
below the water's surface. The creature lived in a chambered
shell, occupying the largest compartments as it grew. Smaller,
unoccupied chambers contained gas, giving the animal buoyancy
and mobility. The soft parts of the ammonite are rarely found
preserved, but the hard shells which these organisms occupied,
are found as fossils all over the world. Widespread distribution
of species make them an excellent guide fossil for studies of
stratigraphic rock layers.
Ammonites first appeared in the fossil record during the
Ordovician Period and died out at the close of the Cretaceous
Period, about sixty-five million years ago. The divisions between
the chambers are called sutures. Sutures are studied to determine
the age of the ammonite. The more intricate the suture patterns,
the more highly evolved and the more recent the ammonite. Ammonites
provide geologists important clues about the geologic age of
deposits in which they are found. Ammonites have been prized
for their beauty since biblical times.
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