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The Ammonites-Index Fossils

Ammonites have been known to mankind for thousands of years. They are the source of many stories and myths. The name of this spiral-shelled cephalopod comes from the Egyptian god Ammon. Ammon was pictured as a man with the horns of a ram protruding from his head. The curled ammonite shells resembled the horns and were called Ammonís Stones or ammonites.

Ammonite Myths
Many cultures throughout history have attributed special powers to this fossil.

In ancient Greece, it was said that if you put an ammonite under your pillow it would cure insomnia and bring good dreams.

The Romans believed that if you put a golden ammonite (pryritized) under your pillow you would have prophetic dreams.

A Brief History of Ammonites
Ammonites first appeared in the Devonian Period. Early curled species had simple septa with a single arc like the members of the class orthocerida. Ammonites of later periods developed septa that had intricate folds called lobes and saddles. They also developed delicate lacey patterns on the outer shell. These patterns along with the shape of the shell and the structure of the septa are how this cephalopod is classified.


Since all living cephalopods (octopus, squid, and nautilus) are predators, we can assume that ammonites were as well. The only living cephalopod with an external shell is the chambered nautilus. It can swim and control its depth. It does this by using the siphuncle. The siphuncle is a tube that connects all the chambers in the shell with the living animal. The nautilus can add or subtract gas in these chambers to control buoyancy.

Ammonite Size
Ammonites have a wide range of size. Specimens have been found ranging from less than a centimeter to 2 meters in diameter. Early ammonites, until the middle Jurassic, were smaller, usually less than 9 inches or 23 centimeters. During the upper Jurassic and lower Cretaceous larger varieties can be found. Titanites found in the south of England can be over 50 centimeters, 2 feet in diameter.

Biostratigraphy
The hard shell of the ammonite was easily fossilized. This, combined with the sheer abundance of this group of cepalopods and its evolutionary duration through several geologic periods, make it a good index fossil. Index fossils help paleontologists and geologists to determine the age of rock layers. This is called biostratigraphy. It works like this. If you find an ammonite from a genus known to be from the Triassic Period, then the rock layer it came from must be Triassic.
To be a good index fossil:


  • It must have wide distribution.
  • There must be a lot of them.
  • It must belong to a group that evolves rapidly.
  • They must be easy to recognize.


Ammonites satisfy all of the above criteria easily.

Extinction of The Ammonites
The end of the Cretaceous Period was also the end of the ammonites. This was about 65 million years ago. Dinosaurs and many other species of plants and animals died out at about this same time. It is thought that a huge meteor colliding with earth caused these mass extinctions.


About the Author: Claudia Mann is a teacher, and a contributor to http://www.fossils-facts-and-finds.com where you can find out more about ammonites. Claudia and her husband own http://www.fossilicious.com, where you can buy ammonites and other fossils at great prices.




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