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)
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.
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).
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.