Ammonites are an extininct group of marine animals also known as Cepahalopods meaning that their head and foot protruded out of the same opening of the shell. They are related to the
extant cuttlefish, squid, and octopus family. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods. Their fossilized shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as "heteromorphs"). The name Ammonite came from their spiral shape as their fossilized shells somewhat resemble tightly-coiled ram's horns. In ancient times they called fossils of these animals ammonis
cornua ("horns of Ammon") after the Egyptian god Ammon who was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in ceras , which is Greek for "horn" (for instance, Pleuroceras) The soft body of the creature occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus the smaller sections of the coil would have floated above the larger sections. Many ammonoids probably lived in the open water of ancient seas, rather than at the sea bottom. This is evidenced by the fact that their fossils are often found in rocks that were laid down under conditions where no bottom-dwelling life is found. The chambered part of the ammonite shell is called a "Phragmocone". The phragmocone contains a series of progressively larger chambers, called "Camerae" that are divided by thin walls called "Septa". Only the last and largest chamber, the "Body chamber" was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. A thin living tube called a "Siphuncle" passed through the septa, extending from the ammonite's body into the empty shell chambers. The ammonite emptied water out of these shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water column. Ammonites may have reached up to 6' in diameter. There are several hundred species than can be found throughout the world. Our ammonite fossils are from the Sahara desert in Morocco (Devonian period 359-416 million), and from Madagascar (Cretaceous 65-146 million)
