Ammonites are an extinct collection of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca. These untimely mollusks had no vertebrae but were protected by a hard curved shell which was made from calcium.

They diverse in size according to the time they lived. Few of the ammonites occurring in the lower and middle fraction of the Jurassic period measured 23 centimetres (9 inches) in diameter. Much larger forms found in the rocks of the upper fraction of the Jurassic and the inferior part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, were often 53 centimetres (2 feet) in diameter. Parapuzosia seppenradensis (the biggest known species of ammonite) of the Cretaceous period of Germany, sometimes reached 2 metres (6.5 feet) in diameter. The biggest documented North American ammonite is 'Parapuzosia bradyi' from the Cretaceous age with specimens measuring 137 centimetres (4.5 feet) in diameter.

The shell of the Ammonites contained numerous air filled chambers, called phragmocones. The animal lived simply in the outer chamber. The opening of the shell is called the aperture. The walls of every chamber are called septa. These walls were penetrated by a ventral tubelike structure called a siphuncle that regulated the air stress, allowing the ammonite to float. Ammonites were speedy touching predators that ate other animals in the sea.

Ammonites were general during the Mesozoic Era, but became extinct during the Cretaceous period. This knowledge helps date rarer fossils of unidentified age. For example, if a new fossil is found in the similar rock layer as an ammonite that is known to have lived only for the period of the Cretaceous period, the new fossil can likely be dated to that similar period. Ammonite fossils are found in enormous quantities and are used as an index fossil.

 

They assist in dating other fossils. The closest living relation of the ammonite is the chambered nautilus. Ammonites were named for Amun (also spelled Ammon), an very old Egyptian god who is pictured as having ram's horns behind every ear (which look like ammonites).

Scientific Classification:

Kingdom: Animalia

Phylum: Mollusca

Class: Cephalopoda

Subclass: Ammonoidea