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)
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