Ammonites are a well known, abundant and diverse group of extinct marine animals. Few groups of invertebrate fossils have received as much attention from both professional palaeontologists and amateur collectors as the ammonites have enjoyed. The ammonites lived during the Jurassic and Cretaceous periods, flourishing in the ancient seas at the same time as the dinosaurs lived on land, and became suddenly and inexplicably extinct at the end of the Cretaceous period. At the same time many other groups of animals, such as the belemnites and the dinosaurs, also became extinct.

There are a number of important features possessed by the shells of ammonites. These are an internally chambered shell, as well as the siphuncle, and the sutures of the shell. These three features are also found in the shells nautiloids and the ancestors of the ammonites.

The siphuncle is a tube which runs through each of the internal chambers of the ammonite near the outside edge of the shell. The thin walls between the internal chambers of the shell are called the septa, and as the ammonite grew it would move its body forward in the shell secreting septa behind it, thereby adding new chambers to the shell. The sutures (or suture lines) are visible as a series of narrow wavy lines on the surface of the shell and are extremely important in the identification and classification of ammonite fossils. The sutures appear where each septa contacts the wall of the outer shell.

The true ammonites evolved from their early ammonoid ancestors the Ceratites at the end of the Triassic period. The ammonoids in turn evolved from the nautiloids early in the Devonian period some 400 million years ago. It is now generally considered that the ammonoids are descended from the bactirids.
The bactirids had a straight or curved shell derived from that of the nautiloids, and the oldest ammonoids share a number of features in common with the bactirids, such as the position of the siphuncle within the shell and the ovoid shape of the initial chamber of the shell.

Ammonites are distinguished from their ammonoid ancestors by the complexity of their sutures, which are simple in species of the Palaeozoic era, and complex in species of the Mesozoic era. The Ceratite Ceratites is a common fossil in the Muschelkalk formation of the Triassic period in Germany, and is also found in France, Spain and Romania.

The ammonoids are a group which includes the ammonites and the goniatites. The ammonoids are classified as part of a larger group - the cephalopods (class cephalopoda), which includes belemnites and nautiloids. There are more than 10,000 known species of fossil cephalopods.
The cephalopods are in turn part of a larger group of animals called the molluscs (phylum mollusca), which includes gastropods and bivalves. The cephalopods include the most biologically advanced and complex forms of all the molluscs.

Ammonites have a worldwide distribution and they evolved rapidly to produce many different species. They are one of the most important groups of fossils for dating rocks from the Mesozoic era. Because of their rapid evolution and widespread occurrence they make excellent zone (or index) fossils, enabling geologists to date and correlate the rocks in which their fossils are found.

The general structure of the ammonite is similar to that of the modern Nautilus, which is a free swimming animal possessing a head with two well developed eyes, arms (or tentacles), and a external shell divided into chambers which are filled with gas making the animal buoyant in the water. However, the soft parts of ammonite bodies are never preserved in fossils.

A feature found in the shells of the modern Nautilus is that of variation in the size of the shell according to the gender of the animal, the shell of the male being slightly larger than that of the female. This variation in shell size according to gender is thought to be an explanation to the variation in size of certain ammonite shells of the same species, the larger shell (called a macroconch) being female, and the smaller shell (called a microconch) being male. This is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in Bifericeras from the early part of the Jurassic period of Europe.

It is only in relatively recent years that the sexual variation in the shells of ammonites has been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in the same rocks. However, these 'pairs' were so consistently found together that it became apparent that they were in fact sexual forms of the same species.

The majority of ammonites have a shell which is a planispiral flat coil (as in the above illustration of Peltoceras athleta), but some have a shell which is partially uncoiled, partially coiled and partially straight (as in Australiceras), nearly straight (as in Baculites), or coiled helically - superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). These partially uncoiled and totally uncoiled forms began to appear during the early part of the Cretaceous period and are known as heteromorphs.

Perhaps the most extreme and bizarre looking example of a heteromorph is Nipponites, which appears to be a tangle of irregular whorls lacking any obvious symmetrical coiling. However, upon closer inspection the shell proves to be a three-dimensional network of connected "U" shapes.  Nipponites occurs in rocks of the upper part of the Cretaceous period in Japan and the USA.

Ammonites vary greatly in the ornamentation of their shells. Some may be smooth and relatively featureless, except for growth lines, and resemble that of the modern Nautilus. In others various patterns of spiral ridges and ribs or even spines are shown. This type of ornamentation of the shell is especially evident in the ammonites of the Cretaceous period.

Because ammonites and their close relatives are extinct little is known about their way of life although a lot has been worked out by using models of ammonite shells in water tanks. Most ammonites probably lived at the surface of the ancient seas, this is suggested by the fact that their fossils are often found in rocks which were laid down under conditions where no bottom-dwelling life is found. Many of them are thought to have been good swimmers with flattened discus-shaped, streamlined shells (such as Oxynoticeras), although some were less effective swimmers and were likely to have been bottom dwellers. Ammonites probably preyed on fishes and crustaceans. Ammonites were themselves preyed upon by marine reptiles (such as Mosasaurs) and their fossilized shells are sometimes found showing the teeth marks from such attacks.

Many ammonoids were able to withdraw their body into the living chamber of the shell and developed either a single horny plate or a pair of calcitic plates with which they were able to close the opening of the shell. The opening of the shell is called the aperture. These plates are termed the anaptychus in the case of a single plate, and are collectively termed the aptychus or aptychii in the case of a pair of plates. The aptychus were of equal in size.

When ammonites are found in clays their original mother-of-pearl coating is often preserved. This type of preservation is found in ammonites such as Hoplites from the Cretaceous Gault clay of Folkestone in Kent, England.

Few of the ammonites occurring in the lower and middle part of the Jurassic period reach a size exceeding 23 centimetres (9 inches) in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic period and the lower part of the Cretaceous period, such as Titanites from the Portland Stone of Jurassic period of southern England which is often 53 centimetres (2 feet) in diameter, and Pachydiscus seppenradensis of the Cretaceous period of Germany which is the largest known ammonite sometimes reaching 2 metres (6.5 feet) in diameter. The largest known North American ammonite is Parapuzosia bradyi from the Cretaceous period with specimens measuring 137 centimetres (4.5 feet) in diameter.

The Cretaceous period Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites, Placenticeras, Hoploscaphites, and Jeletzkytes, as well as many uncoiled forms.

Ammonites fossils become less abundant during the upper part of the Cretaceous period, and none survive into the Caenozoic era.