The trouble with fossils

As Jenny Clack knows, the history of fossils is one of constant revision.

Jenny Clack with fossils
Jenny Clack may not have "sharp elbows", but her painstaking work has transformed our notion of early life on earth. Photograph by Maja Daniels

“Oh, it’s just fallen off,” says Professor Jennifer Clack, prodding Boris with her finger. “That’ll have to be stuck back.”

We are standing in an austere white room at the University Museum of Zoology in Cambridge, dominated by stuffed animals wearing the surprised expression so often induced by overenthusiastic taxidermy, skulls and bones. In front of us is Boris, who died 360 million years ago after a short life spent swimming through weed-choked swamps in the Devonian era.

He now looks like a salamander-crocodile-fish hybrid that has been run over by a truck and encased in stone. After Boris died, his skin and internal organs decomposed, the bloating severe enough to project his pelvis several inches away from the rest of his body. Soon after that, a flash flood left his bones buried in the sediment, where he didn’t do much for the next few aeons until Clack, now curator of the museum, found him in east Greenland in 1987.

The discovery changed how we view a vital period in prehistory: for Boris is an Acanthos­tega (the name means “spiny roof”). This is an early four-limbed creature, or tetrapod, dating from that crucial period of evolution when animals developed arms and legs and came on to the land. Which happened first is a vital question in palaeontology, and Boris, along with the other Acanthostega fossils discovered by Clack – Pop, Patch, Fido, Grace and Rosie – has provided the answer. At the same time, he has prompted a wide and fascinating area of study.

Terrible lizards

The first thing to remember is that the history of fossils is one of constant revision. Palaeontology – the study of ancient life – is fraught with difficulty and its practitioners have to be wary of the dangers caused by scanty knowledge combined with overconfidence.

A few quick examples. There is no such dino­saur as a brontosaurus: the creature so called, discovered by Othniel Charles Marsh, a professor of palaeontology at Yale University, was merely an older example of the juvenile apato­saurus skeleton discovered two years earlier, in 1877. To make matters worse, Marsh found no head to fit the body, so he freestyled what he thought one would look like. According to the science writer Brian Switek, the eventual discovery of the correct head “is what ushered in the big brontosaurus cultural kerfuffle . . . The dinosaur didn’t really become apatosaurus, to members of the public, until the proper head was put on it.” Othniel Charles Marsh (why isn’t anyone called Othniel any more?) also set off another name-wrangle when he designated two types of three-horned dinosaur, tricera­tops and torosaurus, as different genera. There is now heated debate over whether the latter is the mature version of the former. And the torosaurus situation is far from unique. “I think this is where the most change is happening, in terms of museum displays,” Switek tells me. “Some familiar dinosaurs – such as toro­saurus – might disappear.”

Then there is Tyrannosaurus rex. In 1992, the T rex at the American Museum of Natural History in New York had to be dismantled and repositioned; its posture was completely wrong. When the specimen went on show in 1915 – it was the first complete T rex skeleton ever displayed – the prevailing wisdom was that the dinosaur walked upright on two legs, dragging its tail along the ground. Later research showed that such a gait would have dislocated its hips with a single step.

The 1993 film Jurassic Park introduced audiences to the new-look T rex, its body parallel to the ground, the tail used for balance. Yet even Steven Spielberg’s take on the terrible lizard is now outdated; a fossil found in China in 2004 suggests that tyrannosaurs may have had feathers. The velociraptors – “Clever girl!” – that so terrified me as a child certainly did, and far from being human-sized they were more on the scale of large, angry geese.

The examples go on. Go to Crystal Palace in London and see the dinosaur reconstructions there, built in the 1850s, for a glimpse into a galaxy of Victorian inaccuracies. The iguano­dons are the best known. Their thumb-spikes are plonked on their heads, a mistake that has persisted in other depictions of them.

The startling conclusion is this: if you last went to a natural history museum when you were only velociraptor-sized, it’s time to go back. The prehistoric monsters of your childhood might no longer exist.

And if that is the picture with dinosaurs – which appeared only 230 million years ago, and multiple fossils of which from the same types are often found in relatively good condition – imagine how sketchy our knowledge is of the creatures that preceded them. Boris dates another 100 million years earlier than the terrible lizards, and his is the only near-complete Acanthostega skeleton known to science.

Reconstructing roadkill

To explain why Boris is so important to our understanding of evolution, we have to go back to the basement of the Sedgwick Museum of Earth Sciences, a beautiful red-brick building on Downing Street, Cambridge, in the late 1980s. In it stood a young woman called Jennifer Clack.

Always a bright pupil, she had started collecting fossils at the age of ten, scouring the beaches of the “Jurassic Coast” at Lyme Regis but finding nothing more exciting than the odd ammonite.

She had been the first in her family to attend university, studying zoology at Newcastle, but missed out on the first-class degree that would have enabled her to progress to a doctorate. Instead, she moved to Leicester to take a postgraduate course in museum studies, and then to Birmingham to work there. “At that point, I suppose, I saw myself going into a local authority through the museum, being a curator and a naturalist,” she says as we sit in her Cambridge office, its shelves stacked with papers.

A reconstruction of a happy-looking Acanthos­tega is perched on a filing cabinet next to her desk. As she approached her thirties, however, “It became fairly clear to me that this wasn’t my forte and it was also clear to my boss, who was extremely sympathetic and said: ‘Well, why don’t you do some private study?’”

She called her old tutor at Newcastle, Dr Alec Panchen, and asked if there was anything she could help him with. There was. A museum in Bradford had a rare tetrapod called Pholiderpeton that he wanted to study. Could she negotiate its release? As it happened, she could, and she turned her short period of study into a PhD project when she described the fossil’s brain case, which was poorly known at the time.

More research on tetrapods followed after she moved to Cambridge in 1981, Clack painstakingly excavating the fossils from rock with hand tools. Before the advent of computer modelling, working out how the jigsaw creature might have looked when it was alive was an art as well as a science.
“With a bit of luck, you’ve got more than one specimen,” Clack says. “You’ve also got a fairly good idea of how it fits in with the general scheme of things. With that idea in mind, you’ve got to reconstruct the skull.” That becomes even more of a challenge when the fossil is what she describes as “roadkill” – flat and squashed. Today, you can use a computer program to do some of the work, but at that time the method was more low-tech. “In effect, you would make a paper pattern from the individual bones, and then you can put those on to a wax sheet, which is bendable.” Eventually, something like a plausible three-dimensional model would emerge.

The problem was that sometimes the people who put a fossil together “had very set ideas of how it should be”. That was the case with the Swedish palaeontologist Erik Jarvik, who was jealously guarding an important tetrapod fossil called Ichthyostega that Clack dearly wanted to get her hands on. It dated from the Devonian era and was widely regarded as the “missing link” between water- and land-based animals.

The reason she was so keen to examine it was that the sequence of events during this period of evolution was unclear. Jarvik believed that lobe-finned fishes walked on to shore as the planet became warmer and drier, and only developed limbs once they got there. Clack wasn’t so sure, but the only way to decide the question once and for all was to examine the Ichthyostega.

That was a big problem: Jarvik wouldn’t give her – or any other scientist – access to the fossils. He had found Ichthyostega in Greenland in the 1930s, and proceeded to sit on the specimens for the next half-century while he got on with other work. Scientific convention dictated that whoever first uncovered a new creature had first dibs until they had completed their study, so the crucial question seemed destined to remain unanswered for years to come.

I suggest to Clack that this seems like an oddly selfish action for a scientist. “We surmise that he was delegated to do it by his boss, who was extremely difficult to contradict in all sorts of ways,” she says diplomatically. “But he basically would much rather do something else. So, although he felt that he couldn’t give it to anyone else to work on, he didn’t want to work on them himself.”

To make matters worse, the Danish government wouldn’t even let any palaeontologist visit the sites where Jarvik’s Ichthyostega fossils had been found to hunt for more. So Clack decided to try to find new locations in Greenland where tetrapods might be uncovered. She turned to a Cambridge colleague of hers, the geologist Peter Friend, who had done fieldwork in the frozen land in the 1960s and 1970s.

Friend’s team had uncovered many tetrapod fossils while studying the distribution of sediment in the earth. “He had these specimens identified by this guy in London who said, ‘Oh, yes, they’re Ichthyostega.’ End of story, because of this protocol.”

Except that when Clack asked to see the specimens, she found they weren’t Ichthyostega. As she looked at them in the basement of the Sedgwick Museum, she noticed that there were horn-like projections on the back of each skull. They were Acanthostega, another transitional fossil, which at that time was known only from two partial skull roofs.

The priority then became to visit the sites in Greenland where the samples had been discovered to try to find more. In July 1987, Clack and her husband, Rob (who was quite sporting to accompany her, being a computer programmer by trade), spent six weeks camping in 2°C weather, trekking four hours every day from their tents to the fossil bed.

As Clack tells it, there was no “eureka” moment when the couple happened across the Acanthostega fossils in a block of silty sandstone. “We could see some bones sticking through and we could see a cross-section and we could see a series of bumps,” she says. “As the sun glanced across the specimen in Greenland we could see the bones and we thought, ‘Aha! There’s something in there,’ but we had no idea how good it was going to be.”
Back in Cambridge, a preparator went to work, first using a precision jackhammer to remove most of the rock around the fossils. Next out of the toolbox came an air pen, which fires abrasive powder at the specimen. Finally, a mounted needle was used to remove the last specks of what palaeontologists call the “matrix” surrounding the fossil.

As Clack’s postdoctoral student Michael Coates helped prepare the sample that turned out to be Boris, he got quite a shock. At the end of its arm were not five digits, as they had expected, but eight. As Switek notes in his 2010 book on evolution, Written in Stone: “It had long been assumed that the standard number of digits for tetrapods was five, and scientists expected that early tetrapods would either have five digits or would show signs that they had successively added digits until five fingers were formed.” There was some resistance to their findings: some suggested that the researchers had uncovered both hands, each with four digits, and the team had to clear around Boris carefully to show that this was not the case.

The surprises didn’t end there: Acanthostega also had gills and a fishlike tail, and the structure of its wrist showed that it would not support weight. Everything scientists had assumed about the journey from water to land was overturned. Limbs had evolved in the oceans, tetrapods such as Acanthostega using their paddle-like hands to swim through the swamps. When it came to describing this crucial period of evolutionary history, it was back to the drawing board. Or, perhaps more accurately, back to the basement.

“Subsequent to our describing Acanthostega,” Clack says, “people began to look in museum drawers [for fossils] that had been collected in the past and labelled ‘undetermined fish’. Lo and behold, some of them turned out to be tetrapods. A classic example of that is in Latvia, where some of our colleagues were in the Natural History Museum in Riga and were looking in a drawer. They thought, ‘Ooh, that looks like a tetrapod,’ and got an expedition together to go to the locality where that was found. A whole lot more turned up.”

To look at Jenny Clack now, there is no hint that this is a scientist who overturned decades of dogma, almost single-handedly created a field of study and blazed a trail by becoming the first woman in her specialism to be named a fellow of the Royal Society.

She dresses simply and wears her grey hair in a functional bob. Her outfits are enlivened only by jewellery – on the day I visit, she is wearing
a chunk of semi-polished stone on a necklace, but she also has a silver Acanthostega brooch. “She doesn’t have sharp elbows,” a colleague recently told a BBC documentary crew, and there is no sense that she revels in iconoclasm. Perhaps the only hint of a rebellious attitude is that she commutes to work on a motorbike.

“Four” to the floor

What Clack’s story shows is that there are great discoveries still to be made in palaeontology – and not just where you would imagine, in riverbeds and rock faces in distant, windswept lands, but in dusty drawers and teetering boxes that should be looked at afresh. “Talk to any of my colleagues and say, ‘Where is the best hunting ground?’ and they’ll say museum basements,” she says. Specimens unearthed in the 19th century are particularly fertile ground for reappraisal. “The Victorians were very good at finding the material, less good at preparing it. In some cases, it was criminal what they did, in retrospect.” Modern science offers a broad toolkit: not just better optics, but everything from 3D scans to engineering software, which allows researchers to model the mechanics of how an animal walked and stood. (Biomechanics, as this area of study is called, is what put paid to the tail-dragging, upright T rex.)

Clack’s team at Cambridge is now, at last, looking at Ichthyostega, the creature Erik Jarvik found. It might not surprise you that their work, which involves CT scans and computer modelling, is proving that many of the initial impressions of Ichthyostega were wrong. It, too, had limbs that could not support its weight.

“This thing, which is usually illustrated as a sort of four-square tetrapod, with four sturdy legs walking across the sand dunes, is all wrong,” says Clack.

Part of her day is also spent keeping an eye on a series of interesting fossils discovered at five sites across Scotland. These build on finds from the East Kirkton limestone quarry outside Bathgate in West Lothian, Scotland. It is celebrated for yielding valuable specimens from Romer’s Gap, a period immediately after the Devonian era from which few fossils have emerged.

“It was the first time when animals became fully terrestrial,” Clack says. “So when we get to East Kirkton, there they are, everything running about and diverse, all different sizes and shapes, and in the Devonian they were sort of aquatic, and we don’t know what happened in between. There are two locations in the world where you get tetrapods in between. One of them is in Nova Scotia, but the bones are all isolated; we don’t know who they belong to. And the other one is a single specimen from Dumbarton, which is a complete articulated skeleton, which I was fortunate enough to work on.”

This is her new challenge: explaining what happened at the end of the Devonian, when a mass extinction appears to have taken place. What happened to the descendants of Boris, Pop, Patch, Fido, Grace and Rosie? She and the other scientists working on the Scottish finds are seeking funding to explore the whole picture of the period – including the climate, and whether the formation of huge glaciers wiped out many species.
Their research is perhaps our best chance of uncovering the secrets of this era. But it is also possible that the next big revolution in our understanding of prehistory is lying, unloved, in a museum basement.

“Beautiful Minds”, the BBC4 documentary about Professor Clack, is available on iPlayer

Tags:Science