Spirals in Time: The Secret Life and Curious Afterlife of Seashells (25 page)

All of this was later reported by William Leach, curator of zoology at the British Museum, because Cranch himself died of fever, along with most of the crew, and never made it back from Africa. In honour of his lost friend, Leach named the argonaut species
Ocythoe cranchi
, but this was applied only to the soft animals, not the shells they were found in. Many eminent naturalists believed the octopuses didn’t belong with the shell but had killed and devoured the
original occupant before seizing their vessel and sailing off. In their minds the octopuses were nothing but parasites.

Carl Linnaeus had named the shells
Argonauta argo
back in 1758, in the tenth edition of his book
Systema Naturae
, and in 1814 Constantine Samuel Rafinesque assigned the name
Ocythoe antiquorum
to the allegedly parasitic animals that were often found inside. John Cranch’s was a new species of parasitic octopus.

For a long time, a living specimen of the shell-making argonaut itself remained unknown. Presumably they were lurking down in the depths somewhere; perhaps they were some other kind of nautilus. The fact that none had been found wasn’t seen as a major problem, though; after all, chambered nautiluses were very rarely seen alive, but their empty shells were fairly common.

In 1828, English naturalist
William Broderip
reported in the
Zoological Journal
that a French collector in Marseille claimed to have found a
real
argonaut, not a hitch-hiking
Ocythoe
octopus. I can sense Broderip’s eyebrows twitching as he wrote this, and he stayed on the fence, pointing out that much remained to be known before coming down firmly on either side. But he still took a punt that in the long run the octopuses would probably be revealed as pirates, and not the industrious shipwrights of what he called ‘fairy boats’.

The idea of octopuses sailing around in stolen shells may sound like a fanciful Just So Story, but there are some even stranger ideas floating around that have made scientists stop and think. Rather than snatching shells from living species, maybe argonauts hijacked them from far more ancient creatures?

The small collection of living nautilus species is all that remains of an immense cephalopod dynasty. In modern seas, the most common cephalopods are the ones with no external shells, the octopuses, squid and cuttlefish. But in
times gone by it was the shelled cephalopods that reigned supreme. Masses of animals that looked a lot like nautiluses romped through the oceans for hundreds of millions of years. Within that group, the most abundant and diverse of them all were the ammonites, and there were some that looked so eerily similar to argonauts, you might be persuaded they were cast from the same mould. By the late nineteenth century, a distinctly offbeat idea had come to light. What if naked octopuses originally borrowed or stole shells from ammonites? Did argonauts learn how to make shells by copying their ancient relatives?

This theory was first proposed in 1888 by German geologist Gustav Steinmann; it was revisited in 1923 by Swiss palaeontologist Adolf Naef, then again in the 1990s by Zeev Lewy from the Geological Society of Israel. They all imagined the ancestors of modern argonauts to have started out hiding inside empty ammonite shells. Then the argonauts somehow evolved the ability to fix up their borrowed shells, to mend holes and cracks. As the argonauts got better and better at repairing shells they eventually no longer needed a template at all, and could merrily continue shell-making without having to find an ammonite shell first.

Lewy went a step further, proposing that argonauts were in fact scavengers of recently dead ammonites, which he rather charmingly referred to as ‘post-necrotic floaters’. In other words the ammonite shell, complete with dead animal inside, floated to the sea surface and drifted around for a while. Lewy suggested that naked argonaut ancestors laid their eggs inside these post-necrotic floaters, leaving the new hatchlings to slowly eat their dead hosts and ultimately occupy the vacated shell.

To find out if there is any truth in these ideas and see if there is a link between argonauts and ammonites, we should jump back in time half a billion years to see where this all began. Down at the base of the cephalopod evolutionary tree sits a little creature that lived towards the end of the
Cambrian. It was about the size of a pinky toe and wore a slender and slightly bent shell like a wizard’s hat. Charles Doolittle Walcott, of Burgess Shale fame, was the first to describe fossils of these animals (though they were from later deposits), and he named them
Plectronoceras
.

Plectronoceras
is the oldest undisputed cephalopod (strange creatures called
Nectocaris
from the Burgess Shale itself could be cephalopods, although not everyone agrees on that). Their shells were divided into chambers, like nautiluses, and they may have spent much of their lives skipping across the seabed or wafting through shallow seas as part of the plankton. Following on from these modest drifters there were far more impressive, not to mention scarier, cephalopods to come.

Starting around 485 million years ago, the Ordovician was the next major stage in Earth’s history. The planet was a strange place compared to the way things are now. Temperatures were much higher, as were carbon dioxide levels, and most of the landmasses were clumped together into a massive super-continent, Gondwana, but nothing much lived there. Life was still largely confined to the oceans, where there was a mixture of living things that we could recognise today, plus a range of other, bizarre creatures.

Trilobites scuttled across the seabed; bivalves and brachiopods stayed put as they sifted the water for food; gastropods ambled past fronds of red and green seaweeds and colonies of coral. Above the seabed, early chordates called conodonts wriggled their eel-like bodies and gnawed at their food with the sharpest teeth that ever evolved; floating through the water were colonial creatures called graptolites that looked like delicate, saw-toothed tuning forks. For all of these creatures, one of the most dangerous things they were likely to encounter in Ordovician seas was an enormous shelled cephalopod.

The unassuming Cambrian cephalopod lineage went on to flourish in the Ordovician. They evolved into masses of
new groups; some were tightly coiled, others had shells as straight as pencils. Incomplete remains have been found of gigantic straight shells from a creature named
Cameroceras.
Estimates of their full size range up to an astonishing 10 metres (more than 30 feet), as long as a double-decker London bus. These were formidable beasts, like a primeval apparition of a Colossal Squid, the main difference being that these ancient creatures lived inside the longest seashells ever to exist.

It’s generally thought that
Cameroceras
may have spent a good deal of time resting close to the seabed, pulling itself along with a cluster of arms and scooping prey into its mouth. Other straight-shelled cephalopods would have hung in the water with their heads down, grabbing prey from the bottom. Some evolved counterweights at the ends of their long shells and swam horizontally. Like giant spears, they could have shot through the oceans in pursuit of prey. Whichever way you look at it, the Ordovician saw the rise of the cephalopods.

Towards the end of this period, the super-continent Gondwana drifted towards the South Pole, giant ice sheets spread across the land and Earth fell into a very deep ice age. Sea levels dropped, and continental shelves were drained of their shallow seas, depriving much marine life of its habitat and triggering a mass extinction. Over half of all marine invertebrates were wiped out, but cephalopods were among the survivors.

For tens of millions of years, cephalopods waxed and waned many times. Throughout the Silurian and into the Devonian periods, they went through repeated pulses of decline but always picked themselves up and carried on, recovering their abundance and diversity. It was in the early part of the Devonian, around 400 million years ago, that a series of important new branches sprouted in the cephalopod evolutionary tree. There were the Nautilida or nautilids that led on to the modern nautiluses. The coleoids showed up
too, which eventually gave rise to the living octopuses, cuttlefish and squid. The third major lineage of cephalopods to emerge in the Devonian went on to produce some of the most supreme sea creatures of all time: the ammonites.

Chronoscopes and thunderstones

If you fancy getting your hands on your very own ancient, extinct creature I’d recommend looking for an ammonite. Fossil ammonites are hugely abundant and widespread, not to mention very lovely objects. I have several ammonites that were found and given to me by Kate, my geologist sister, who knows only too well my soft spot for interesting things from the sea. My favourite in this little collection is an intricate, tightly coiled shell covered in delicate ridges, and just the right size to cover up with my thumb. It got trapped in a layer of black silty mud that eventually turned to mudstone and became part of the crumbly cliffs of Kimmeridge Bay on England’s south coast. This animal swam through the seas 150 million years ago and now sits on my desk, where from time to time it helps to straighten out my sense of perspective on the world, and of time passing.

Because they’re so common and easy to find, fossil ammonites have been wending their way into human lives for thousands of years, sometimes without people even realising. Walk through the Grand Arcade shopping centre in my home town of Cambridge, England and look down, and you’ll spot ancient spirals in the polished limestone tiles beneath your feet. Long before anyone knew their true origins, and way before they began appearing in shop floors, people across the globe found these strange coiling stones and wondered what they were.

In Europe, fossil ammonites were often called snakestones, with accompanying legends to explain how they were made. Often it was a story about a saint, who went around turning real snakes into stone then hurling them off cliffs.
Snakestones were widely believed to cure snakebites and all sorts of other conditions, from human impotence to cramp in cows.

Ancient Romans believed they would see into the future if they slept with a golden ammonite under their pillow. The Blackfoot people of North America thought ammonites looked like sleeping bison and called them buffalo stones; finding one before a journey was a good omen. Black ammonites from the Gandaki River in the Himalayas are called shaligrams. They are worshipped in monasteries and temples as manifestations of the Hindu god Vishnu, and people on their deathbeds drink water steeped in these sacred stones to free them of their sins.

Similar beliefs surround belemnites. These extinct relatives of the ammonites were coleoids, along with octopuses and squid, and while they were quite squiddy in their external appearance they had an internal, bullet-shaped shell. Fossil belemnite shells, known as thunderstones, were thought to be created when thunderbolts struck the ground, and they too were used as a cure for snakebites, as well as protecting a house from getting hit by lightning when they were placed on a windowsill. In Swedish folklore thunderstones held strong magical powers that guarded against evil; they were thought to be candlesticks used by supernatural creatures called
vättar
that live under the floorboards and cause trouble if the house isn’t kept tidy (in some versions of the story they are distant relatives of Santa Claus). In eighteenth-century England, fossil belemnites were ground down and used as an ointment for horses with sore eyes. In Scotland, the traditional name for them was botstone; people would drop one in a horse’s water trough to treat a case of worms.

Bountiful fossil ammonites have also been put to practical use. In Victorian Britain, they were dug up and used to make the world’s first artificial fertiliser. As urban populations grew and more mouths needed feeding, scientists discovered
that phosphate was a key ingredient for growing better crops. Expensive bird droppings, rich in phosphate and known as guano, were imported from Peru at substantial cost. Animal bones from knacker’s yards, shavings from bone-handled knife factories, mummified Egyptian cats and allegedly even human remains from European battlefields were all ground down and sprinkled onto arable fields. Then a source of phosphate was found much closer to home. Buried deposits of fossilised bones, mixed in with assortments of ancient animal teeth, claws, shells and the droppings of extinct marine reptiles were found to be an excellent source of phosphate. The concoctions came to be known as coprolites, from Greek words for dung and stone, even though only some of it was actually petrified poo; everything else technically should be referred to as pseudo-coprolites or better still, phosphatic nodules. In the mix were ammonites; after they died, the calcium carbonate in their shells was replaced with calcium phosphate from seawater.

A shallow Cretaceous sea that used to cover south-east England winnowed fossil ammonites from older rocks and swept them into dense piles. It was these ancient relics that triggered a coprolite mining rush and saw open-cast mines appear across the country. Great fortunes were made in digging up and processing coprolites, in particular around the city of Cambridge, where almost all of Britain’s raw phosphate came from.