Some of the most numerous fossils that have survived into the modern age are those coiled shells left behind by the prehistoric ammonites. When Pliny the Elder first examined the remains of these ancient marine cephalopods, he called them ammonis cornua (‘horns of Ammon’) because the Egyptian god Amun was typically depicted wearing ram’s horns. And from that we get the name ammonite.
Around a hundred million years ago, for reasons still largely unknown to us, the great ammonite dynasty began to dwindle. The very last of them died out in the extinction event 65 million years ago that also heralded the demise of the dinosaurs.
Today, almost all cephalopods (including the octopuses, squid and cuttlefish) have lost their external shells, allowing them to be fast, agile predators, but one small group – the odd ones out in the family – have retained them. They are the nautiluses. They, like many of the extinct ammonites, have spiral-shaped shells, with the fleshy animal itself protruding from the bottom. Whereas ammonite shells were corrugated or covered in spikes, though, the shell of a nautilus is mostly smooth.
The nautilus itself does not take up all of the shell – most of it is comprised of the same air-filled chambers that the ammonites possessed, allowing the animal to float near the surface of the ocean. As the nautilus grows, more chambers are added to provide sufficient buoyancy for the increasing weight of the animal. If it wishes to descend deeper into the ocean, it can flood these flotation chambers to adjust its buoyancy accordingly. But although a nautilus seems to merely float and wobble aimlessly through the water, sometimes even bumping into things, it can move much 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 at considerable speed.
Despite the physical similarities between the nautilus and the extinct ammonites, it is not actually descended from them – in fact, it lived alongside them in the ancient seas – and nor is it closely related to other living cephalopods. Firstly – as we have already mentioned – it has kept its external shell when all other modern cephalopods have done away with theirs altogether. Secondly, the nautilus has a simpler brain and nervous system, and much simpler eyes. In fact, these lensless ‘pinholes’, which project a very hazy and dim image onto its retinas, are thought to be the simplest eyes of any large living animal, only really useful for telling the difference between light and dark (the nautilus is much more reliant on its sense of smell instead).
Meanwhile, a nautilus’ tentacles – up to ninety of them – are not suckered like those owned by other cephalopods, although they are ribbed, allowing the animal to grapple with its prey. And finally, a nautilus is capable of living for up ten times as long as most octopuses and squid (sometimes up to twenty years), which is probably due to its slow metabolism and energy-saving methods of locomotion.
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’. But the fact that it has remained unchanged for around 70 million years means that there’s still life in this strange shelled creature yet.