More About Ammonites

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

The shell of the Ammonites contained many air filled chambers, called 'phragmocones'. The animal lived only in the outer chamber. The opening of the shell is called the 'aperture'. The walls of each chamber are called 'septa'. These walls were penetrated by a ventral tubelike structure called a 'siphuncle' that regulated the air pressure, allowing the ammonite to float. Ammonites were fast moving predators that ate other animals in the sea.

Ammonites were common during the Mesozoic Era, but became extinct during the Cretaceous period. This knowledge helps to date rarer fossils of unknown age. For example, if a new fossil is found in the same rock layer as an ammonite that is known to have lived only during the Cretaceous period, the new fossil can likely be dated to that same period.

The closest living relative of the ammonite is the 'chambered nautilus'. Ammonites were named for Amun (also spelled Ammon), an ancient Egyptian god who is pictured as having rams horns behind each ear (which look like ammonites).