Fossil Information Sheet F008
Ammonites
Perhaps the most widely known fossil, possessing the typically ribbed spiral formed shell.
The name comes from its appearance: it resembles a ram's horn.
In Egyptian mythology the God Ammon looked like a man with horns like a ram.
The ancient fossil was considered Ammon's stone, thus inheriting the name Ammonite.
These creatures lived in the seas between 240 and 65 million years ago when they became extinct along with the Dinosaurs.
They belonged to a group known as Cephalopods whose closest modern day living relatives are actually the Coleoids (Squid, Octopus, Cuttlefish, etc.) rather than the Nautiloids (the Nautilius).
Cephalopods are characterised by a hollow shell divided into chambers by septa.
As the animal grew new chambers were added behind the head chamber.
A small tube called the Siphuncle links the chambers.
The chambered interior of the shell is referred to as the Phragmocone and in life this contained gasses which enabled the ammonite to regulate its bouyancy.
Ammonites moved by jet propulsion expelling water through a funnel-like opening to propel themselves.
Nautiloids had simple Septa with a single arc. The Ammonites developed Septa that had intricate folds called lobes and saddles.
Some Ammonite fossils bear intricate patterned detail on their outer surface called "Sutures" these are very helpful in distinguishing the many different species of Ammonites: Goniatites, Gastrioceras, Ceratites, Lytoceras, Phylloceras, Asteroceras, Harpoceras, Hildoceras, Stephanoceras, Perisphinctes etc.
These different types of Ammonite fossils are also widely used for dating rocks.
Typically they lived for around two years although some survived beyond this and grew very large.
Their modern day equivalent, the Nautilus, possess many of the same characteristics.
In comparison, however, the Ammonite preferred warm shallow waters and consequently had a higher metabollic rate and as such could reach a larger size more quickly than the modern day Nautilus which lives in deep cold water.
Ammonites are displayed in whole natural form (on the left a specimen from Morocco mounted on a brass plinth) or in "sliced" halves displaying the most beautiful internal structures and colours.
Some of the most beautiful internally coloured pieces come from Madagascar (see on the right) but examples are found all around the world.
In some cases the ammonite becomes pyritised.
The example on the left, a "Rainbow Pyritised Ammonite", has a whole variety of mineral ingress as well as pyrite.
The pyrite gives the metalised effect on the surface of the split halves, whereas the other minerals lead to the "rainbow" of colours within the chambers.
The other, on the right, is striking example of a "Fire Black Ammonite" from the Volga River showing ingress of black and honey calcite.
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