Ammonites are an extininct group of marine animals
also known as Cepahalopods meaning that their
head and foot protruded out of the same opening
of the shell. They are related to the extant cuttlefish,
squid, and octopus family.
They are excellent index fossils, and it is often
possible to link the rock layer in which they
are found to specific geological time periods.
Their fossilized shells usually take the form
of planispirals, although there were some helically-spiraled
and non-spiraled forms (known as "heteromorphs").
The name Ammonite came from their spiral shape
as their fossilized shells somewhat resemble tightly-coiled
ram's horns.
In ancient times they called fossils of these
animals ammonis Cornua ("horns of Ammon")
after the Egyptian god Ammon who was typically
depicted wearing ram's horns. Often the name of
an ammonite genus ends in ceras , which is Greek
for "horn" (for instance, Pleuroceras)
The soft body of the creature occupied the largest
segments of the shell at the end of the coil.
The smaller earlier segments were walled off and
the animal could maintain its buoyancy by filling
them with gas. Thus the smaller sections of the
coil would have floated above the larger sections.
Many ammonoids probably lived in the open water
of ancient seas, rather than at the sea bottom.
This is evidenced by the fact that their fossils
are often found in rocks that were laid down under
conditions where no bottom-dwelling life is found.
The chambered part of the ammonite shell is called
a "Phragmocone". The phragmocone contains
a series of progressively larger chambers, called
"Camerae" that are divided by thin walls
called "Septa". Only the last and largest
chamber, the "Body chamber" was occupied
by the living animal at any given moment.
As it grew, it added newer and larger chambers
to the open end of the coil. A thin living tube
called a "Siphuncle" passed through
the septa, extending from the ammonite's body
into the empty shell chambers. The ammonite emptied
water out of these shell chambers. This enabled
it to control the buoyancy of the shell and thereby
rise or descend in the water column. Ammonites
may have reached up to 6' in diameter.
There are several hundred species than can be
found throughout the world. Our ammonite fossils
are from Madagascar (Cretaceous 65-146 million)
only