MINERAL OF THE MONTH: 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.

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

Leave a Reply

Your email address will not be published. Required fields are marked *