The nautilus and the ammonite

This article was inspired by a poem in which an ammonite and a nautilus travel the world’s oceans for millions of years, until they are finally separated by extinction, and is based on a talk I gave on HDGS Members Day, on 18 July 2010.

The nautilus and the ammonite

The Nautilus and the Ammonite were launch’d in storm and strife;

Each sent to float, in its tiny boat on the wide, wide sea of life.

They roam’d all day, through creek and bay, and travers’d the ocean deep;

And at night they sank on a coral bank, in its fairy bowers to sleep.

And the monsters vast, of ages past, they beheld in their ocean caves;

And saw them ride, in their power and pride, and sink in their deep sea graves.

Thus hand in hand, from strand to strand, they sail’d in mirth and glee;

Those fairy shells, with their crystal cells, twin creatures of the sea.

But they came at last, to a sea long past, and as they reach’d its shore,

The Almighty’s breath spake out in death – and the Ammonite liv’d no more.

And the Nautilus now, in its shelly prow, as over the deep it strays,

Still seems to seek, in bay and creek, its companion of other days.

And thus do we, in life’s stormy sea, as we roam from shore to shore;

While tempest-tost, we seek the lost – but find them on earth no more!

GF Richardson (1851)

The ammonite

Ammonites belong to a group of sea animals known as cephalopods, which today includes their relatives the octopus, squid, cuttlefish and nautilus. It was nearly 500mya that the first cephalopods appeared in the ancient seas. From primitive organisms, they gradually evolved into highly successful species, with the ammonites becoming most prolific during the Mesozoic, 250 to 65mya.

The ancient Greeks gave the name ‘ammonite’ to this fossil, because its coiled shape resembled the horns of the ram-headed Egyptian god, Amun.

An ammonite’s shell has internal chambers (Fig. 2), which increase in size as they rotate around a central point. The largest chamber, with its open aperture, would have contained the ammonite’s body. As the animal grew bigger, it secreted minerals to enlarge the aperture, while, at same time, sealing off part of the shell behind its body, thereby creating a new chamber. The chambered interior of an ammonite is known as the phragmocone.

Fig. 2. Section through an ammonite.
Fig. 2. Section through an ammonite.

Most shells have about five or six whorls (rotations) and it has been estimated from fossil evidence that each whorl took from between four months to three years to grow.

A tube-like structure, called the siphuncle, linked the chambers by passing through the upper part (venter) of the coiled shell. A recently sealed chamber would contain seawater, but this was gradually replaced by gases (mainly nitrogen, oxygen and carbon dioxide), which diffused into the chamber through osmosis. Once filled with gas, a chamber generally stayed that way – although small amounts of water could re-enter through the siphuncle for fine tuning of buoyancy at various depths.

Some ammonites have been found with small calcite plates called aptychi. In the past, it was assumed that each aptychi formed a cap which closed the opening of the shell to protect the animal from predators. However, more recent research seems to indicate that, in fact, they were part of the jaw apparatus.

Although the fossilised shells of ammonites occur in huge numbers, almost nothing is known of their soft parts – apart from possible outlines of digestive organs and ink sacs which, in very rare cases, have been preserved. While no evidence of tentacles has been found, it can be assumed that ammonites were similar to modern cephalopods, such as nautiloids and squids in this respect.

Much of what we know about ammonites has been worked out by studying their shells and by using models of them in water tanks. Occasional muscle scars preserved on the shell interior suggest that ammonites probably moved by forcing water through a funnel-like opening to propel themselves in the opposite direction (that is, through jet propulsion).

The Nautilus and the Ammonite Fig. 3
Fig. 3. Spiral (heteromorph) ammonite.

Some ammonite fossils reveal intricate suture patterns, which formed beneath the external shell wall and locked each chamber together like pieces of a jigsaw. Sutures are often visible if the shell has been worn away, either by erosion or through artificial polishing. As well as serving to lock the chambers together, the construction of complex sutures probably provided extra strength to the shell when diving to deeper water. As every species of ammonite has its own unique suture pattern, this can provide a very useful means of identifying particular specimens.

Those with thick-ribbed shells were likely to have been slow-moving bottom-dwellers. Fossil evidence indicates that their diet included molluscs and crustaceans, which lived on the seafloor. These ammonites were themselves preyed upon by larger predators and have been found showing teeth marks from such attacks. However, mollusc borings in ammonite shells are sometimes misidentified as teeth marks. Having strongly-ribbed and thick shells, sometimes with protective spines, would certainly have increased their chances of survival. They may also have escaped from an attack by squirting ink, much as modern cephalopods do.

The Nautilus and the Ammonite Fig. 4
Fig. 3. Spiral (heteromorph) ammonite.

Ammonites with flattened, discus-shaped, streamlined shells are thought to have been fast-moving hunters, which fed on various marine creatures, including fish and even their own kind. An attack probably involved stalking their prey, then rapidly extending tentacles to grasp the victim, which would be torn apart by strong, parrot-like jaws. These are believed to have been located between the eyes at the base of the tentacles.

The majority of ammonites have a shell that forms a flat coil, known as a planispiral (Figs. 5 and 6). However, some have shells that are almost straight – such as Hamites from the Gault Clay. These partially, and also totally, uncoiled forms began to diversify during the early part of the Cretaceous.

The Nautilus and the Ammonite Fig. 5
Fig. 5. Pyritised ammonite, Echioceras raricostatum, from Lyme Regis.

Other species, known as heteromorphs, evolved with shells coiled into a helix shape (Fig. 3), resembling a Turritella gastropod. Perhaps the most bizarre looking example of a heteromorph is Nipponites (Fig. 4), which is found in Japan and the USA. It appears to be a tangle of irregular whorls, without any obvious symmetry. However, upon closer inspection, the shell is actually a three-dimensional network of connected ‘U’ shapes. Some theories suggest that ammonite shells evolved with various shapes and sizes because this played an important part in their social and mating behaviour.

During their evolution, the ammonites faced no less than three catastrophic extinction events. The first occurred at the end of the Permian (250mya), when only 10% of species survived, but these managed to expand throughout the following Triassic. However, after another huge extinction event at the Triassic/Jurassic boundary (200mya) only one genus remained. Despite this, during the next 150myrs, their numbers increased and diversified once more. Finally, in the disastrous extinction event at the end of the Cretaceous, 65mya, all of the ammonites disappeared. The ash and dust from a meteor impact and volcanic eruptions would have blotted out sunlight around the earth for months or even years. It has been estimated that this would have killed off much of the planktonic plants and zooplankton in the sea – possibly the food on which the tiny ammonite offspring depended for their survival. Another factor may have been the huge increase in predatory fish during the Upper Cretaceous – a theory supported by teeth marks on ammonite shells. Fossil evidence also indicates that ammonite species may have been in decline for around 30myrs before this extinction – an event which proved to be the last straw for them.

The Nautilus and the Ammonite Fig. 6
Fig. 6. The ammonite, Hildoceras bifrons, from Whitby.

When ammonites died and sank to the seafloor, they were gradually buried in accumulating sediment. Bacterial decomposition of soft parts often resulted in the precipitation of minerals, which formed a hard concretion around their shells. Most fossils have been preserved as a result of such mineralisation. The minerals include quartz and iron pyrites (Fig. 5), but the most common is calcite, a mineral dissolved from limestone by groundwater and transported in solution. In a process known as diagenesis, original aragonite shell material is gradually replaced by recrystallised calcite to produce a detailed replica. Sometimes, when a shell disintegrates completely within a short time, the empty space is filled with sediment, which eventually hardens to become a fossil cast with no internal structure.

Ammonites may be found in many sizes – ranging from millimetres to metres. The Portland stone near Swanage contains Titanites, which is often 60cm in diameter. One of the largest recorded specimens, measuring up to two meters across, was found near Münster in Germany.

There are species that display iridescence in their shells, although this would not have been visible during the ammonite’s life. The colours are created by refraction of light and the microscopic structure of the fossil shell material. When such fossils are found in clays, their original mother-of-pearl shell may be preserved as an iridescent coating. This effect is often found in specimens from the Gault Clay of Folkestone, while other beautiful examples (Psiloceras) can be seen in the Jurassic shales at Watchet in Somerset.

Today, fossils of ammonites are abundant in the Jurassic limestones of Dorset and Yorkshire, and the Cretaceous chalk of Kent and Sussex. They make ideal index fossils for dating rocks, because certain species lived within well-defined time periods. It is often possible to link the rock layer in which they are found to specific geological periods and therefore geological maps can provide excellent guides to the best locations for collecting.

In medieval times, fossilised ammonites were thought to be petrified snakes, and were called “snakestones” or “serpent stones”. According to a famous legend dating from the seventh century, a local Saxon abbess, named Hilda, wanted to build an abbey on the cliffs overlooking the sea at Whitby. Unfortunately, the site was plagued by snakes, but on Hilda’s command, they hurled themselves over the cliffs. After writhing and curling up as they fell through the air, the snakes hit the beach and immediately turned into stone. Hilda was later canonised to become Saint Hilda. Since then many generations of local people sold ‘St Hilda’s serpents’ to visitors. The absence of heads on the snakestones was difficult to explain, so the enterprising inhabitants simply ‘restored’ the heads by carving them on the ammonites.

In the seventeenth century, three snakestones were incorporated into the Whitby coat of arms and, in 1808, the legend of St Hilda was immortalised by Sir Walter Scott in his poem, Marmion.

Thus Whitby’s nuns exulting told –

How that of thousand snakes, each one

Was changed into a coil of stone,

When holy Hilda prayed.

To commemorate the legend of St Hilda, a particular genus of ammonite, Hildoceras (Fig. 6), was named after her.

The nautilus

The name, ‘nautilus’, is derived from the ancient Greek word for ‘sailor’, perhaps because it reminded them of a ship bobbing in the sea. Today, the nautilus is often described as a “living fossil”, because it has remained virtually unchanged for more than 400myrs.

Its shell is formed from a nacreous aragonite, with a white iridescent inner layer (mother-of-pearl). Like the ammonite, a nautilus retains its original shell throughout life and creates larger chambers as it grows. The rotating chambers increase in number from around four, when hatching, to 30 or more in adults. This produces a structure, which is well known as one of the finest examples in nature of a logarithmic spiral (Fig. 7). However, unlike the ammonite, which has its siphuncle at the top edge of its shell, the siphuncle of the nautilus rotates through the middle of the shell.

The Nautilus and the Ammonite Fig. 7
Fig. 7. Section through a nautilus.

The nautilus is a predator that feeds mainly on shrimp, small fish and crustaceans, but, because very little energy is expended in swimming, it only needs to eat about once a month. It usually has up to 90 short tentacles – more than any other cephalopod. These are arranged into two circles around the mouth and have a very strong grip. Instead of pads and suckers, the tentacles have ridged surfaces, which enable them to stick to their prey. It also has powerful parrot-like jaws that are capable of slicing through the hard exoskeletons of arthropods.

To swim, a nautilus uses jet propulsion by forcing water into and out of the living chamber with a funnel called the hyponome. Despite having a bulky shell, it is capable of making rapid darts and turns, as well as being able to hang motionless in the water. It is able to achieve this because, like the ammonite, its shell has gas-filled chambers that can be adjusted to give neutral buoyancy. However, a nautilus shell cannot cope with extreme pressures. At depths greater than about 800m, it would almost certainly implode.

The animal has excellent camouflage in the water. When seen from above, the shell is darker in colour and marked with irregular stripes, which help it to blend into the dark water underneath. The underside is almost completely white, making the animal almost invisible when viewed from below.

Unlike many other cephalopods, the nautilus does not have good vision. Its eye structure is highly developed, but lacks a solid lens. It has a simple ‘pinhole’ eye, which is constantly open to the environment. Instead of vision, it is thought to use smell as the primary sense for finding food and locating potential mates.

Despite their similarity in lifestyle and design, the fact is that the nautilus survived while the ammonite disappeared in the last mass extinction, 65mya. It appears that, while the shallow-water ammonites were affected by catastrophic events that killed off the plankton, some nautilus species were able to survive by moving to deeper water to find food.

Today, the nautilus belongs to a group comprising seven living species and a single sub-species, which is found only in the Indian and Pacific Oceans. Most of them never exceed about 20cm in diameter, although one species from Western Australia may reach nearly 27cm.

They usually inhabit the deep slopes of coral reefs at about 300m and move towards the surface at night to feed, mate and to lay eggs. Females spawn once a year and attach the fertilised eggs to rocks in shallow waters, where the eggs take eight to 12 months to develop and hatch. A nautilus may live for over 20 years – an exceptionally long life-span for a cephalopod.

Unfortunately, the ‘great survivors’ are now facing their greatest challenge – humans. Thousands of nautiluses are caught for their shells each year, which are sold as souvenirs or carved into jewellery or buttons. Their numbers are declining and sadly one population in the Philippines has already been wiped out.

After the nautilus has survived all that earth can throw at it over 400myrs, it is up to us to make sure it isn’t pushed into extinction by the most voracious predator of all.

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