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Fall 2010 – Ammonite Fossil

The mineral of the month is the ammonite (Ammonoid) fossil. Ammonites are an extinct group of marine invertebrate animals from the Cephalopod class. These fossilized mollusks are more closely related to living cephalopods such as octopuses and squid, than they are to the modern nautilus, which has a similarly shaped shell. Two pictures of ammonite fossils are below. I took the first photo and Tom Shearer took the other two photos (Figures 135 & 136 in the new agate book).


Ammonites are excellent index fossils. Depending on the species of ammonite found in a rock layer, geologists can specify the geologic time period for that layer.

The name ammonite was inspired by the spiral shape of the fossilized shells, which somewhat resemble tightly-coiled rams' horns. The Egyptian god, Ammon, was typically depicted wearing rams' horns.

Eight different orders of ammonites are known to have existed, ranging from 400 million to 65.5 million years ago. They ranged in size from a fraction of an inch to over 7 feet in diameter. After being born, they fed on plankton and quickly assumed a strong protective outer shell. They also grew quickly with the females growing up to 400 percent larger than the males because they needed the extra space for egg production.

Because ammonites are extinct, little is known about their way of life. Their soft body parts were very rarely preserved in any detail. It is thought that depending on the species, they lived in a variety of ocean environments. Some probably lived in open water, while others survived at the bottom of the ocean. It is also believed that ammonites may have avoided becoming a predator's dinner by squirting ink, much like modern cephalopods.

The soft body of the organism occupied the largest segment of the shell at the end of the coil. The smaller sections were walled off which allowed the animal to maintain its buoyancy by either filling the chambers with gas, or emptying the sea water out of these chambers. Thus the smaller sections of the coil would have floated above the larger sections. As it grew, it added newer and larger chambers to the open end of the coil.

Ammonite Reconstructed

In medieval Europe, fossil ammonites were thought to be petrified coiled snakes. These fossils were often called "snakestones" or "serpentstones."


Mineral of the Month Archives

May 2007: Rainbow Fluorite

June 2007: Lake Superior Michipicoten Agate

July 2007: Labadorite

August 2007: Rain Flower Agate

Fall 2007: Malachite

December 2007: Nepheline Syenite

January 2008: Native Copper

February 2008: Amazonite

March 2008: Lake Superior Agate

April 2008: Shadow Agate

May 2008: Apohpylite

June 2008: Ocean Jasper

Summer 2008: Marra Mamba Tiger's Eye

September 2008: Mohawkite

October 2008: Mexican opal

November 2008: Prehnite

December 2008: Picture Jasper

January 2009: Sea Shell Jasper

February 2009: Polychrome Jasper

March 2009: Selenite Desert Rose

Spring 2009: Coyamito Agate

July 2009: Obsidian Needles

August 2009: Goethite

September 2009: Banded Iron Formation

Fall 2009: Fairburn Agate

March 2010: Fossilized Dinosaur Bone

April/May: 2010 Kentucky Agate

June 2010: Nantan Meteorite

July 2010: Mookaite Jasper

Aug/Sept 2010: Polyhedroid Agate

Fall 2010: Ammonite Fossil

September 2011: Petoskey Stones

Spring 2011: Petrfied Wood

Winter 2011: Argentina Condor Agate

January 2012: Mary Ellen Jasper

March 2012: Mexican Crazy Lace Agate

June 2012: Moqui Marbles

September 2012: Chlorastrolite Greenstone

March 2013: Jacobsville Sandstone

August 2013: Unakite

November 2013: Skip-an-Atom Agate

April 2014: Tiger's Eye

September 2014: Black Corundum

February 2015: Condor Agate

June 2015: Petoskey Stone

November 2015: Slag

June 2016: Lake Superior Copper Replacement Agates

March 2017: Chert

July 2017: Kona Dolomite

December 2017: Septarian Nodule

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Gitche Gumee Museum.
E21739 Brazel Street
Grand Marais, Michigan 49839


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