A Blast from the Past
Some of the most numerous fossils that have survived into the modern age are those coiled, ridged shells left behind by ammonites. Indeed, many rocks between 66 and 200 million years old seem to comprise little else but the mineralised remains of these prehistoric cephalopods. It seems that for a vast period of time, ammonites were among the most abundant of all marine creatures.
Their shells, which had flotation chambers within them for bouyancy, came in a huge variety of shapes. Flat spirals, if their remains are anything to go by, were extremely common, but others were shaped like corkscrews or trombones. Some species would begin growing their shell in a tight spiral but then straighten them out during later growth phases. One type, Nipponites, had a shell that appears to be a tangle of wildly irregular whorls lacking any obvious symmetrical coiling. And ammonites also varied greatly in size: some were smaller than your little fingernail, while at least one species was over 2 metres in diameter.
But around 100 million years ago, for reasons still largely unknown to us, the great ammonite dynasty began to dwindle. It was once thought that the very last of them died out in the cataclysmic extinction event 66 million years ago that also heralded the demise of the dinosaurs, although we now suspect that one or two varieties lingered on for a little longer.
Today, almost all cephalopods (including the octopuses, squid and cuttlefish) have lost their external shells, allowing them to be fast, agile predators. One small group, however, has retained them. They are the odd ones out in the family – the nautiluses.
Nautiluses were once very common, but only six species survive today. They, like many of the extinct ammonites, have spiral-shaped, compartmentalised shells, and what was once the molluscan foot has divided into a bristling mass of up to 90 short, grasping tentacles that are very sensitive to taste. But whereas ammonite shells were corrugated or covered in spikes, the shell of a nautilus is mostly smooth, pearly, and lustrous.
The nautilus does not take up all of its tightly coiled shell. Most of it is comprised of the same sealed, pressurised internal chambers that the ammonites possessed, with the animal itself only occupying the outermost and largest compartment. At the start of its life, the shell consists of just a few chambers, but as the nautilus grows, more and more chambers are added to provide sufficient buoyancy for the increasing weight of the animal. By carefully adjusting the air-to-water ratio within these radiating chambers using a tube called the siphuncle, the nautilus can achieve neutral buoyancy, reducing the energy needed to swim and allowing it to support the weight of a big shell. If it wishes to descend deeper into the ocean, it can flood these flotation chambers.
Normally, a nautilus seems to hang in the water as if attached to an invisible wire, floating and bobbing aimlessly along. But it can also move faster when it wishes to by sucking water into a cavity in its mantle and then expelling it with a muscular contraction through a siphon, jet-propelling itself backwards in a jerky, bobbing style. By doing this, though, it can’t see where it is going and may bump into things.
Despite the physical similarities between the nautilus and the ammonites, it is not actually descended from them. In fact, it lived alongside the ammonites in the ancient seas around 200 million years ago. Nor is the nautilus closely related to other living cephalopods. As already mentioned, it is the only modern cephalopod to have kept its external shell. It also has a much simpler brain and nervous system, and it shows none of the intelligence possessed by octopuses, which can solve puzzles and indulge in play. The nautilus can, however, live for up to 10 times longer than most octopuses and squid (sometimes up to 20 years), which is probably due to its slow metabolism and energy-saving methods of locomotion.
The stalked, unblinking eyes of the nautilus are worth mentioning because they are extremely primitive. These crude, lens-less ‘pinholes’, which project a very dim and hazy image onto its retinas, are thought to be the simplest eyes of any large living animal in the world. Because the nautilus is much more reliant on its sense of smell, the eyes are only really useful for telling the difference between light and dark.
It seems like the nautilus is the great-granddad of the cephalopod family. It’s near-blind, somewhat unsteady when it moves, and refuses to let go of the ‘old ways’. It is truly a mollusc out of time. But the fact that it has remained superficially unchanged for around 70 million years, scraping through a major extinction event that wiped out its ammonite relatives, means that there’s still life in this strange shelled creature yet.