The History House - Ammonites

AMMONITES

Here is a wonderful image of a collage of ammonites and to find this would be any fossil hunter’s dream come true.

Maybe tomorrow one of us will be so lucky.

Fossil ammonites are the only remains of an extinct group of cephalopods. Cephalopods are fast moving predatory marine invertebrates. Their existence on earth lasted for about 330 million years. Living members of this group include cuttlefish, squid, octopus, and the Nautilus, but only the Nautilus has a fully developed shell.

Click on image for a larger view. Click on image for a larger view.

Ammonites were nektonic, or free-swimming creatures, which lived in the water below the surface. Ammonites lived within a chambered shell, occupying the largest compartments as it grew, which is called the body chamber. The individual chambers are created by separating the interior of the shell with partitions, or walls, called the septa. Smaller, unoccupied chambers contained gas, providing buoyancy and mobility.

Gas and liquid exchange occurs through the septa walls dividing the chambers by a tube called the siphuncle. 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. Like the dinosaurs, they too fell prey to the mass extinction that occurred during this time period. 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 important clues about the geologic age of deposits in which they are found. Ammonites have been prized for their beauty since biblical times. Here are some of the worlds most prolific ammonite deposits.

DEVONIAN AMMONITES OF MOROCCO
Morocco has vast deposits of Devonian Limestone which date back three hundred fifty million years. Ammonites found today in Morocco once flourished in a warm shallow sea which covered what is now the Sahara Desert. As the shells of the creatures accumulated on the sea floor, they were buried by sediments.
Over the ages, they were transformed into stone by physical, and chemical processes. Today, they are quarried from marble, or calcium carbonate deposit. The stone is carved and polished by Moroccan craftsmen to reveal a rare glimpse of ancient life on earth and are often sold commercially.

JURASSIC AMMONITES OF GREAT BRITAIN
Great Britain is rich in fossil ammonites from the Jurassic Period. High and crumbling cliffs at Lyme Regis and Charmouth are formed of shale and beds of limestone. Ammonites of all sizes are common. Small ammonites preserved in pyrite occur in shales and are found loose on the beach.
Near Whitby, there are beds of nodules which often contain ammonites. The Fradingham Ironstone is composed of lenses of the different ironstone laid one on top of another. The calcite replacement of the ammonites from this location is a striking contrast of white against a gray mudstone.

CRETACEOUS AMMONITES FROM THE UNITED STATES
A number of species of cretaceous ammonites are found in the Pierre and Bearpaw Shales of Montana and Fox Hills Sandstone of South Dakota. Some were quite large, up to 30 inches in diameter. Though it was once believed that they were a favorite food of the Mosasaurs, which frequently dined on this Cretaceous creature, and fossil records are now questioned about whether indentations in the shells of the ammonite were from Mosasaur.

Cretaceous ammonites have extremely intricate suture patterns. They are often found with a colorful nacreous shell coating which reflects beautiful color flashes when coated with a clear lustrous resin. Some species flash bright red, green and lavender colors and are prized as a gemstone called
"Ammolite".

Other species have soft opal colors much like "mother of pearl". The intricate suture patterns are revealed when the nacreous shell coating is removed, so there is some consolation in finding an ammonite without this original coating.

Ammonites belong to the group of ammonoids which includes the goniatites. Ammonoids evolved from the nautiloids early in the Devonian period some 400 million years ago. The nautiloids are part of the group of animals called the cephalopods, which also includes ammonoids, belemnites and nautiloids. To learn more about belemnites, goniatites, and nautiloids, click on the icons below for that subject.

Belemnites

Goniatites

Nautiloids

To see additional ammonite images that show a variety ammonite exteriors, just click on the icon below.

To a collection of ammonites from Texas, just click on the icon below, and take a peek.

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Fossil ammonites are the only remains of an extinct group of cephalopods. Cephalopods are fast moving predatory marine invertebrates. Their existence on earth lasted for about 330 million years. Living members of this group include cuttlefish, squid, octopus, and the Nautilus, but only the Nautilus has a fully developed shell.
Click on image for a larger view.	Click on image for a larger view.
Ammonites were nektonic, or free-swimming creatures, which lived in the water below the surface. Ammonites lived within a chambered shell, occupying the largest compartments as it grew, which is called the body chamber. The individual chambers are created by separating the interior of the shell with partitions, or walls, called the septa. Smaller, unoccupied chambers contained gas, providing buoyancy and mobility.

Gas and liquid exchange occurs through the septa walls dividing the chambers by a tube called the siphuncle. 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 belong to the group of ammonoids which includes the goniatites. Ammonoids evolved from the nautiloids early in the Devonian period some 400 million years ago. The nautiloids are part of the group of animals called the cephalopods, which also includes ammonoids, belemnites and nautiloids. To learn more about belemnites, goniatites, and nautiloids, click on the icons below for that subject.
Belemnites	Goniatites	Nautiloids