Ammonite

Ammonites are a group of extinct marine organisms of the order mollusca and suborder Ammonoidea. The Time period of ammonoids is from the Devonian to the Cretaceous. Jurassic and Cretaceous ammonoids are called ammonites. Since ammonites have all the characteristics of an index fossil, including a wide geographical distribution, a limited temporal distribution, and independence from a specific facies, they have occupied a large part of paleontological studies in all parts of the world.

Though soft tissues are poorly preserved, The rapid evolution of distinguished genera, their subsequent extinction after a few million years, and the excellent preservation of unique shell features make ammonites an excellent index fossil

Ammonites were predatory mollusks that resembled squid. These cephalopods had eyes, tentacles, and spiral shells. Ammonites first appearing in the fossil record 240 million years ago, descending from straight shelled cephalopods. The last lineages disappeared 65 million years ago at the end of the Cretaceous.

The largest ammonite fossils known are those of Parapuzosia seppenradensis, which measure up to 180cm (nearly 6 feet) across

Ammonites shells are found with a variety of spiral types. Most are planispiral, though some have helical forms or no spiral at all. The phragmacone pertains to the chambers which control buoyancy and are separated by septae.

The septae form suture patterns which have an elaborate, folded design. The sutures are on the inside of the shell, where the septa meet the inner wall. The beautiful patterns would have been hidden behind the protective, outer part of the shell on the living animal.

In the clam chambers of these organisms, there is a gas, a mixture of gases similar to atmospheric air but absent Oxigen, and it is composed primarily of nitrogen. The siphon (membrane tube) pumps water in and out of the shell. The siphon has veins and arteries, and its tissue is capable of selecting water and salts.