Spirals in Time: The Secret Life and Curious Afterlife of Seashells Page 3
How it all began
The oldest known fossil shells date from the Cambrian period, around 540 million years ago, with the so-called ‘small shelly fossils’. This collection of minute marine fossils crops up in various places around the world. Among them are puzzling tube-like creatures that might be sponges or corals as well as masses of titchy shells, one or two millimetres (about one-sixteenth of an inch) long, that look rather like molluscs as we know them today. In the mix are shells with tightly twisted coils; some are conical like a Christmas elf’s hat and some have twin shells like a clam. Most palaeontologists agree that these must have been molluscs, although a few remain cautious, pointing out that, although we have their shells, these fossils don’t tell us enough about the animals that made them for us to be sure what they really were.
Alongside these tiny shelled creatures, a troupe of enigmatic unshelled animals were creeping across the Cambrian seabed. Following their discovery more than a century ago in the world’s most famous fossil site, academic arguments have raged over the identity of these strange animals and whether any of them were in fact the very earliest molluscs.
On 30 August 1909, American geologist Charles Doolittle Walcott was riding his horse alone in the Yoho National Park in the Canadian Rockies when he made a ground-breaking discovery. He was looking for fossil trilobites – ancient arthropods that looked like giant, ornate woodlice – but on that day he came across some very unusual fossils. Several months later, in a letter to a geologist friend, Walcott referred to these new fossils as ‘very interesting things’, which was putting it mildly. In the coming years, he returned to the same spot in the Rockies many times, travelling by railway, horse and foot and in total collecting 65,000 extraordinary fossils, the likes of which no one had ever seen before. The site came to be known as the Burgess Shale.
Among his discoveries, Walcott found bizarre animals with hosepipes for snouts, terrifying creatures with massive claws and covered in enormous spines, plus all manner of shrimpy, crabby, wormy creatures that look very little like any living species. Nevertheless, he was convinced these were just strange versions of animals we know of today. In 1911, Walcott found one particular fossil at the Burgess Shale, a part of which had already been found elsewhere. Twelve years previously, Canadian palaeontologist G.F. Matthew had found a single, ribbed spine while fossil hunting in the Wiwaxy Peaks in the Rockies. He called it Wiwaxia. Walcott was the first to find fossilised remains of the complete animal. He decided it was a type of bristly worm known as a polychaete, a member of the annelid phylum. But it didn’t have much in common with any living polychaete worms. Wiwaxia looked more like a slug fitted out with a suit of overlapping body armour, and with elongated knife blades sticking up in two rows along its back.
Walcott found hundreds of Wiwaxia, including two-millimetre-long spineless specimens and larger ones, up to five centimetres (two inches) in length. And yet, peculiar as they were, Wiwaxia and the other fossils found in the Burgess Shale didn’t raise much more scientific interest for the next 50 years. Walcott is perhaps best remembered now as the man who didn’t quite realise what astonishing things he had found.
It was only in the 1960s that palaeontologist Harry Whittington from Yale University decided to take another look. Whittington had already revolutionised the world of trilobite studies when he uncovered silica specimens, fossils made essentially of glass, that revealed dainty details of their mysterious lives. His interest in trilobites led him to the Rockies, where he reopened excavations of the Burgess Shale deposits and began a monumental task that would continue for the rest of his life.
Whittington took up a professorship at the University of Cambridge where, along with his research students Derek Briggs and Simon Conway Morris, he reassessed the Burgess Shale fossils. Together they opened a new window into the origins of animal life. It was through their work that the concept of the ‘Cambrian explosion’ took hold, where a plethora of complex animals appeared in a sudden flurry (although more recently the pace and duration of these changes have been questioned). Evolution seemed to be tinkering with the possibilities for life.
Among the piles of new discoveries and reinterpretations, it was Conway Morris who re-examined Wiwaxia and decided that it wasn’t a polychaete worm after all. Inside Wiwaxia’s mouth he found two rows of backward-pointing teeth that he thought were rather familiar. They looked to him like the rasping radula (a feature of many modern molluscs, which we will return to shortly).
While he thought the rest of Wiwaxia’s body was too strange to win it a formal place within the mollusc phylum, Conway Morris interpreted the fossil as being a common ancestor of the group. Was this odd, spiny slug the precursor to mollusc life? Little did Conway Morris know at the time, but debates over the true identity of Wiwaxia had only just begun.
Since then, Wiwaxia has suffered from an identity crisis as people argued over whether it was a worm, or a mollusc, or something else. Nick Butterfield, also at Cambridge, waded in on the discussions early on and pushed Wiwaxia back worm-wards. He pointed out that Wiwaxia’s sclerites (the ribbed scales of its body armour) were built more like a worm’s bristles; what’s more, its mouthparts could have been split and arranged in two parts on the sides of its head, a distinctly worm-like trait.
Wiwaxia isn’t the only problematic proto-mollusc of the Burgess Shale fossils. In the original excavations Walcott found a single fossil of Odontogriphus, a flattened, oval creature that grew up to 12.5 centimetres (close to five inches) long, with a hardened covering across its back. It had a small, circular mouth on its underside that seemed to be adorned with radula-like chompers just like Wiwaxia.
Conway Morris looked at Odontogriphus again in the 1970s and concluded it was a common ancestor to the worms, molluscs and brachiopods. Then in 2006, after nearly 200 more specimens were found, Jean-Bernard Caron at the Royal Ontario Museum published a paper proudly claiming Odontogriphus for the molluscs. Caron and his colleagues also drew a close connection between these and another, even older fossil called Kimberella. Discovered in the 1960s in the Ediacara Hills in South Australia, the flattened egg-shaped fossils of Kimberella were first thought to be jellyfish. Then trace fossils were found that suggested they spent their lives not pulsing through open water but creeping backwards across the seabed, scraping up food with tiny teeth. But Kimberella’s teeth have never been found, so no one knows whether their snail-like scuff-marks really were made by a radula.
Some striking recent advances in our understanding of molluscan ancestry came from looking at these ancient fossils in a completely new way. For his Ph.D, Martin Smith put fossils inside a scanning electron microscope and captured images of electrons bouncing off atoms deep inside the specimens. This revealed their inner structure in unrivalled detail and convinced him that Wiwaxia and Odontogriphus were not worms. Smith worked out that both of them shed their teeth and grew new ones throughout their lives, and occasionally they would swallow them; a few fossils have teeth lodged in their guts. The bigger the animal, the more teeth it had, and each tooth swivelled relative to its neighbours. All of this, and more besides, lent weight to the idea that these fossils had molluscan kinship.
In a 2014 paper, Smith provided more support for the idea that Wiwaxia was an early mollusc. He studied a handful of Wiwaxia fossils that seemed to have a single foot, like modern slugs and snails. But part of the puzzle remains unsolved. Smith hasn’t yet been able to decipher exactly where to place Wiwaxia on the tree of life, although he has at least narrowed things down. One possibility is that it belongs among the molluscs that don’t have a single shell, the aculifera (including the chitons, solenogastres and caudofoveates). These weren’t the earliest molluscs to evolve, so it would mean Wiwaxia wasn’t a mollusc ancestor. Alternatively, Wiwaxia could be placed on a lower branch, as a stem group to all the molluscs. This would make it a precursor to the mollusc phylum, closer to molluscs than to any other modern group, but not quite a mollusc.
The concept of
stem and crown groups has gained interest in palaeontological circles over the last 15 years. Crown groups are living species that share key characteristics (along with an ancestor that they all have in common, plus any extinct species that also evolved from that same ancestor). Stem groups are extinct species that have some but not all of those characteristics of the crown group. They are aunts and uncles to the crown group, taxonomically speaking.
This approach is helping palaeontologists to make sense of the jumble of strange animals that emerged around the time of the Burgess Shale. Many of these in-betweenie fossils could be stem groups to living phyla rather than members of fully formed phyla themselves, living or extinct. It underscores the fact that key characteristics defining a particular group of living things didn’t all evolve at once but rather gradually, step-by-step, over time. It’s the difference between going to a department store to buy a whole outfit compared to assembling a look from a mixture of vintage hand-me-downs, old favourites and new shoes.
Contemplating stem groups in the deep past reveals that the boundaries drawn between phyla are perhaps somewhat arbitrary. Looking at living species, it is plain to see that molluscs are very different from, say, annelids or echinoderms. But as palaeontologists peer further back through time and in greater detail, those boundaries become blurred.
If Wiwaxia is a stem-group mollusc, it would suggest that the radula, sclerites and a single foot were among the earlier characteristics to appear in the mollusc lineage. But it leaves an important unanswered question.
Which came first, the mollusc or the shell?
By the Late Cambrian, most of the major mollusc groups had evolved. There were indisputable bivalves, gastropods, cephalopods and chitons; scaphopods came along a while later. All of them became more abundant and diverse in the following geological period, the Ordovician. A few other mollusc groups came and went through the eons, including the now-extinct rudists; back in the Jurassic and Cretaceous these twin-shelled molluscs formed the foundations of teeming tropical reefs, similar to the coral reefs of today.
All things considered, the mighty mollusc lineage has been going for at least half a billion years, and in all that time these super-abundant, super-diverse animals have kept some secrets to themselves. We still don’t really know how the different groups – the bivalves, cephalopods, chitons and so on – are related to each other, and we don’t know for sure which of them came first.
Following years of research, including comparisons between living animals and more recently the arrival of genetic techniques, experts are still wrangling over molluscs. Like a pack of playing cards, the mollusc groups keep being shuffled around; should we put all the red cards together, the kings and queens in one place, should diamonds go next to hearts because they’re the same colour or do they belong with the spades because they have a point at the top? Scientists keep grabbing the pack of mollusc cards from each other and moving things around.
The wobbliness of the mollusc family tree (or phylogeny) and the fact that it keeps changing shape has important implications for the way we understand evolution and the variety of life on Earth. It matters, for example, to people studying the evolution of complex brains whether cephalopods and gastropods are closely related or not, because both these groups have well-developed nervous systems; did these systems evolve twice, independently, or just once in a shared ancestor?
These questions, and many more, are tackled by a recent trio of studies that delve deep into the mollusc phylogeny. The three studies involved large research teams led by Kevin Kocot from Auburn University in Alabama, Stephen Smith, now at University of Michigan, Ann Arbor, and Jakob Vinther, now at Bristol University in the UK. The methods they all used were incredibly complex, with the outcomes depending on many things, from the choice of mollusc species and out-group (the non-mollusc species used as a comparison) to the way the data are analysed. All three teams used similar DNA sequencing techniques (using nuclear protein-coding genes, not ribosomal genes as in earlier studies), but the results they throw up don’t all agree.
One conclusion that all three studies do settle on is the identity of the aculifera; they all confidently proclaim that chitons, solenogastres and caudofoveates do indeed belong together on the same branch of the mollusc family tree.
A radical outcome from one of these studies is the relationship between cephalopods and gastropods. Traditionally, these two classes were clustered together as sisters, offshoots from the same junction on the mollusc family tree. But rather than bringing them together, some of the latest genetic findings have separated the octopuses from the snails. Cephalopods could instead be more closely allied with the mysterious monoplacophorans, the deep-sea molluscs that were thought to be long extinct. Morphological studies in the past had linked these two groups, based on their fossils having a similar arrangement of internal organs, and now genetic studies have breathed new life into this idea. The gastropods are bundled, quite confidently, in with the bivalves and the scaphopods (although the scaphopods continue to be a pain in the neck to identify; we simply don’t know enough about them to be sure where exactly they fit in). If this is correct then it suggests that molluscs evolved complex nervous systems on at least four separate occasions: big news for neurobiologists.
And what about the identity of the last common ancestors of all the molluscs? Did they have shells or not? This remains the subject of hot debate. Vinther and his team argue that the earliest molluscs were conchifera (the animals with single shells) and that the aculifera (without single shells) evolved later. On the other hand, both Kocot and Smith’s papers keep things ambiguous: maybe it was the aculifera that evolved first, maybe it was conchifera. For now, we just don’t know.
Jumping forward to the present day and casting an eye around the modern molluscs, we see no single character that all of them share, but instead there is a grab-bag of body parts; some species have them all, others only a selection. These include the radula, a muscular foot and the sclerites. Add a set of internal organs shaped like feathers called ctenidia (or gills), plus a hard shell made by a layer of soft tissue known as the mantle, and you have the basic ingredients for making all living molluscs.
This collection of mollusc body parts has proven to be incredibly malleable and adaptable. Rather than a Lego set, complete with all the specific parts to build a Star Wars Millennium Falcon, think of a box of modelling clay that can be made into anything your imagination allows. Similarly, each mollusc body part has been reconfigured, reshaped and repurposed over time by natural selection, allowing molluscs to wildly alter their appearance and way of life.
In effect the mollusc lineage has been riffing on a theme for half a billion years. They have been trying out experiments in how to eat and avoid being eaten, how to move about, and how to have sex and make more of themselves. This opened the way for molluscs to move into new habitats, to fill a huge range of ecological niches and ultimately to evolve into hundreds of thousands of species. Molluscs are supreme shape-shifters, and it’s this versatility that could explain their roaring success, as we can see by looking in turn at each of the main body parts.
All the better to rasp/chew/stab/harpoon you with
Peer into a mollusc’s mouth (preferably with the aid of a microscope) and be prepared for a terrifying show of fangs. They may be small, but they are some of the most complicated teeth on the planet.
The radula – a bristly tongue made of a protein called chitin – is covered in rows of tiny teeth, laid out across a conveyor belt that creeps ever forwards, with new teeth made at the back and old, worn-out ones falling out at the front. A single radula can have anywhere between a handful and many hundreds or even thousands of teeth, and each mollusc species has a unique arrangement of gnashers. Gastropods, in particular, have really gone to town with their teeth. They’re organised into groups with names that sound like Dr Who aliens; watch out for the rhipidoglossans, the hystrichoglossans and the toxoglossans. I’d like to imagine that molluscs grin at eac
h other to identify themselves, but of course they don’t.
The precise shape and configuration of the teeth on the radula determines what molluscs can eat. Some radulas allow for quite simple but varied diets, sweeping up loose diatoms, slurping strings of seaweed like noodles or scraping at rocks and boulders covered in green slime. Limpets rasp microbes and seaweed sporelings from rocks, like a cat licking a bowl of frozen milk. The reason their teeth don’t instantly shatter when they do this is because they’re made from the strongest known biological material. A 2015 study found that limpet teeth, made of an iron-based mineral called goethite, are up there with the very strongest artificial materials. Limpets could chew holes in bulletproof jackets, if they wanted to. At low tide, you can see the zigzag marks they scrape across rocks and you can even hear them eating; quietly place a stethoscope on a rock near one of these little herbivores, and you should be able to make out the intermittent, sandpapery scratching as the limpet gathers its food.
Other vegetarian molluscs have evolved more specialised radulas, including the sacoglossans, a group of sea slugs that suck. They use their teeth to pierce the cell walls of plants and seaweeds, then suck out the sap inside. Many are incredibly picky eaters, feeding on a single species and, like fine gourmet diners, they have cutlery to match. Their teeth can be serrated triangles, sharp blades or shaped like wooden clogs; they’re adapted to pierce particular types of underwater growth, from leathery kelp to crusty seaweeds. With their specialised teeth and diets, sea slugs divide up habitats, allowing lots of species to evolve and coexist.
Snaggle-toothed radulas become frankly terrifying in molluscs that evolved to be hunters. Many have teeth like flick knives that stand on end, locking in place during attacks, then folding safely away when not in use. A few years ago, an eerie white slug was found in a garden in Cardiff, Wales. It was a species new to science and experts had a shock when they saw its teeth: it was the UK’s first predatory slug. Most land slugs, though, much to the annoyance of gardeners, are herbivores. And at a mere two centimetres (half an inch) long, the new slug is not exactly a sabre-toothed tiger, but it’s no less scary if you happen to be an earthworm.