Spirals in Time: The Secret Life and Curious Afterlife of Seashells (4 page)

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.