Introduction

Ammonites were around from the beginning of the Jurassic period to the end of the Cretaceous period. They were common in the oceans of the Earth for 143 million years, before disappearing at the K-T¹ boundary extinction event 65 million years ago. They ranged in size from less than half an inch to over nine foot across. They were almost certainly predators, going by the behaviour of their one living relative, the Nautilus. Their fossil remains are numerous, and can be found in almost every country. For a page full of enlargable thumbnail pictures, the following Google Image Search is set to ammonites.

A Classification

Ammonites are named after the Egyptian deity Ammon – often represented as having ram's horns (from the Latin, Cornu Ammonis – Horn of Ammon). Ammonites have a passing resemblance to ram's horns, but describe a spiral, not a helix. There is a temptation to describe them as flattened snails, but this would be misleading, as some species resemble nothing less than a cannon ball.

Although the shells of some ammonite species may superficially resemble those of a snail, the creature that lived inside the shell had more in common with a cuttle-fish or squid than a land or marine snail. The shell was also employed very differently. Ammonites are cephalopods (chambered shells), whereas snails are gastropods (non-chambered shells). The body of the snail inhabits all of the shell; the body of an ammonite inhabited only the front section, or 'chamber' of the shell. Both are molluscs, which defines them as soft-bodied invertebrates (no back bones). Beyond those comparisons, they are very different creatures indeed.

Appearance and Underlying Structure

It is unknown whether ammonites were coloured or not, as colour disappears during the fossilisation process. Their only living relative, the Nautilus, has a cream coloured shell with cooked lobster red splashes and banding. It is almost certain that the hundreds of species of ammonites displayed colours on their shells, but without hard evidence it is nothing more than speculation.

The nautilus is a predator, and hunts much in the same manner as the squid and cuttlefish. All three are capable of moving at speed and grasping prey in their tentacles. It is a fair to assume that ammonites hunted in much the same way. The major difference is in the shell. Both ammonites and nautiloids have chambers and, as has been mentioned earlier, only inhabit the front chamber. The rest of the chambers contain a mixture of gas and fluid, and are connected to each other by a tube which runs between the chambers. This is called the 'siphuncle'. The purpose of the siphuncle is to distribute the gas and fluid between chambers, adjusting the buoyancy of the animal. Manipulation of the gas and fluid enables the animal to rise or descend rapidly through water, which is crucial for both hunting and escape from predation.

Mating Habits

Although often found singly, ammonites also frequently turn up in large numbers, suggesting that the individuals died at the same time. Mass mortality sites are not a sign of extinction – they occur at many points during which the same species of ammonite is known to have thrived. It is possible that – similar to squid – they died off after mating. However, this researcher has a slab of stone covered in ammonites of different sizes, all of the same species. The difference in size goes beyond that of sexual dimorphism – there are specimens a couple of inches across down to less than a quarter of an inch. These periodical 'die offs' remain something of a mystery.

Mass mortality sites often have fossilised wood running through them. This is explained by trees felled by storms being washed out on the tide. The trees become water-logged and sink, settling on the sea-bed. The ammonites, having died for whatever reason, were carried by currents and caught in the branches of the sunken trees. This scenario is especially plausible considering the fact that a lot of Jurassic/Cretaceous trees exuded a sticky resinous sap which could have easily been strong enough to capture the relatively light shells.

Why Are They Extinct?

Nobody knows for sure why these animals are extinct while their close relative continues to live. Nautiloids, and presumably ammonites, have a larval stage. This is not unusual – a lot of sea creatures reproduce in the same manner. The larvae of many different species often swim together, collectively making up a large proportion of the ocean's plankton. Nautiloid larvae, however, do not have a surface stage. They remain fairly deep, and this may be why they survived the K-T extinction event. It is possible that ammonite larvae were surface dwellers, and succumbed to the same fate that wiped out the vast majority of species during those calamatous times.

Where Can They Be Found?

The most famous place to find ammonites (in the UK at least) is Lyme Regis. They can be found in limestone nodules on the beach and at the foot of the cliffs. Caution is advised – during heavy rains the cliffs turn into a black gelatinous mud, and slides are common. The tides in this area are also notoriously fast, trapping the unwary with embarrassing frequency.

Different Kinds of Fosilisation

The Cliffs below Black Venn at Lyme Regis often yield pyritised ammonites – that is, the ammonite has turned into iron pyrities, or fool’s gold. Mineralisation is crucial for fossil preservation; indeed, it is never the creature that is represented by the fossil, but a mineral version of what that animal would have looked like. Over time, as the pre-fossil animal decays, the spaces left behind in the sediment in which it lies are replaced by the minerals that are most common to that area. Hence opalised ichthyosaur and plesiosaur skeletons from the Moon Plain, near the opal mining town of Cooby Pedy, Australia. Although the opal itself wouldn’t be worth that much due to impurities, the rarity of the finds guaruntee a substantial price. In addition to opalised ammonites, there are irridescent ammonites (the crushed beds of which are often sold as ‘ammolite’, now a semi-precious stone), pearlescent ammonites (looking for all the world like Mother of Pearl), and ammonites composed of calcite which has preserved the ‘suture’ lines of the chambers, making the ammonite appear as if it were covered in fractals.