by Matthew Norton
There is so much diversity in the oceans. A place where life has found millions of ways to live, build, fight and avoid dying for as long as possible. With so much out there, it’s inevitable that some species, or even groups of species, will be overlooked to the extent that we sometimes forget they exist at all. In my last article, I briefly mentioned how cuttlefish are sometimes overshadowed by their squid and octopus cousin. What I failed to mention is that there is a fourth group of cephalopod molluscs living in today’s oceans. The Nautiloids (also called Nautilus).
Nautiloids have been around for about 500 million years, long before the coeloids (the cephalopod sub-group that octopuses, squid and cuttlefish belong to) emerged. At the peak of their success, it is thought that there were around 2,500 different species of nautiloid. But over time, the world changed and new competition emerged. The mass extinction event at the end of the Cretaceous period (around 66 million years ago) was a particularly difficult time for the nautiloids. While whole animal groups were wiped out, the nautiloids pulled through, but never recaptured their former glory. Today, there are only six species left.
A few hundred million years ago, naming all the nautiloids in the sea would have been a much more difficult task.
Nevertheless, nautiloids are still recognisable as cephalopods thanks to some tell tale features. They have strong tentacles, sharp beaks, well developed eyes and the ability to jump around by firing jets of water. But the one obvious feature they have which other living cephalopods don’t is a shell on the outside. External shells would have been a must have for all cephalopods at one time in earth’s history, but while the coeloids chose to abandon their shells, the nautiloids were more stubborn in the ways of their ancestors.
Even so, the nautiloid shell is an impressive feat of natural engineering, particularly on the inside where it is separated into a series of chambers The outermost chamber is huge and built for housing the animal itself, while all the others are built for holding gases and liquids to keep the nautiloid afloat. To allow them to float up and sink down as needed, the system is regulated by a tube called the siphuncle. This tube passes through each shell chamber, sucking up or dishing out gases and liquids.
The ocean is undoubtedly full of wonders. Some dominate this world as they have never done before while others belong to a world that faded away a long time. But that doesn’t mean these ‘living fossils’ have stopped evolving, only that they have chosen to refine existing designs rather than invent revolutionary new ones. Nautiloids have done this well enough to linger on while extinction has claimed so many others. But even for these survivors, the threats they face today are unprecedented, even for our planet. And it would be a terrible shame if we lost those last few links we have to the ancient seas.
From a human perspective
At the risk of contradicting what I was just saying about their obscurity, nautiloids have still managed to fascinate us from time to time. Their empty shells have long been moulded into jewelry and revered as natural curiosities while the very name Nautilus has inspired scientific journals, poems, science fiction aliens and Russian rock bands. In the novel Twenty thousand leagues under the sea, the Nautilus was the name of the vessel commanded by Captain Nemo as it travelled the oceans. A story that ended ironically (spoiler alert) with this particular nautilus being attacked by its most formidable cousin. A giant squid.
Returning to the real world, the ancient origins and design of nautiloids have made them very useful for understanding how life worked in the ocean long before we were around to take notes. We do also have the fossilised remains of sea creatures to work from, but often it’s only the hard shells and bones that come through the fossilisation process intact. We can work out a lot from these preserved hard parts, but we can also look to living surrogate species to fill in the blanks. In the case of those last few living nautiloids, they have been used as a surrogate species to study ammonites.
Ammonites were another group of cephalopods with outside shells who flourished at around the same time as the nautiloids. But while the nautiloids managed to just about hang on through the end of the Cretaceous period, the ammonites were completely wiped out. All we have left are the fossilised remains that have captured the attention of scientists for over 150 years. More than enough time and study to notice that the preserved shells of ammonites and the fresh shells of nautiloids are incredibly similar down to the way they’re divided into chambers on the inside.This strongly suggests that ammonites used the comparmentalised shells to float, just like our living nautiloids.
But despite these similarities at face value, nautiloids are not a perfect match for ammonites. They’re not even their closest cousins with a series of subtle differences revealing that ammonites are more closely related to the coeloids (octopuses, squid etc.) than they are to nautiloids. One such subtle difference is in the design of the siphuncle, the tube that carries gas and liquid between shell chambers. In nautiloids, the tube is calcified (i.e. turned to bone) on the inside and soft on the inside, while in the ammonites and early coeloids (before they went shell-less) the tube is hardened inside and out. It’s a small modification, but it could have been what saved the nautiloids while the ammonites were going extinct.
Working out how prehistoric animals lived is always going to be a tricky business. The fossil record is patchy, the use of living surrogates is far from perfect and evolution does like to give us some bizarre twists along the way. It’s a testament to the skill and patience of paleontologists, and researchers in related fields, that we are not completely clueless about earth. Furthermore, we are forever discovering more fossils and developing new techniques to probe deeper into these fossils and into the lives they once lived.
A study that analysed a particularly well preserved Baculites fossil (not pictured) with x-rays partially matched the structure of its jaws to that of three living molluscs. All of whom feeding on tiny planktonic animals that float around in the water.
The remains of such planktonic creatures were even found within the jaws of the Baculites. While it is possible that these tiny animals were cleaning or scavenging for food in the ammonite’s mouth, balance of probability suggests that this ancient mollusc had food stuck between its teeth when it died.
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Our world under the waves 
Another well scripted chapter on the World beneath the Waves. So vast, powerful and beautiful. Great to receive in my in box amongst the less positive news.
I have a large ammonite or May be it’s a nautiloid… you can confirm sometime.
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