SEEN through the right geological lens, the bucolic countryside near Chirnside, a village in south-east Scotland, becomes a tropical swamp. The rocks divulge a picture of a sweltering and soggy landscape, tangled with all manner of tree ferns, horsetails and 30-metre-high clubmosses that look like giant scaly asparagus spears.
Here, 350 million years ago, off the edge of a muddy bank, a pair of eyes poked above the water. They belonged to a newt-like creature with a broad head, a wide mouth full of needle-sharp teeth and a long tail. It also boasted four limbs, with which it shuffled awkwardly onto the bank.
This amphibious vertebrate, nicknamed Tiny by its discoverers, might be the most important fossil you’ve never heard of. It lived through a time for which our records are sparse, but when one of the most significant transitions in life’s history was taking place. This was the era in which fish-like things hauled themselves out of the water for new life on land, setting the stage for the rise of amphibians, reptiles and mammals like us.
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Tiny isn’t the only recent discovery challenging our view of this transition. Where once we imagined that a few sturdy pioneers exchanged their fins for limbs, took a gulp of air and never looked back, now we see a haphazard process that relied as much on shifty, swelling eyes as the anatomical prototypes of limbs.
So far, we’ve uncovered a number of flagstones on the evolutionary path from fish to four-legged land animals, or tetrapods. In rocks dating from 375 million years ago, the end of the Devonian period, palaeontologists have found a fossilised shoal of fishy creatures that document the evolution of fins into the limbs and fingers that would eventually carry them onto land. Fleshy-finned fish related to today’s lungfish mark the start of the transition, with fossils like Panderichthys from Latvia and the 375-million-year-old Tiktaalik from Ellesmere Island, Canada, demonstrating how fin bones were modified into the rudiments of our own appendages.
Fast forward about 10 million years and vertebrates seem even better suited to wandering ashore. There was Acanthostega, roughly salamander-shaped and 60-centimetres long, with well-defined limbs and eight fingers on each hand, and the larger Ichthyostega, with its seven digits. Although recent studies suggest it would have been more comfortable in water, Ichthyostega was capable of dragging its body along the mud banks. At this point, early tetrapods had limbs, fingers and the ability to breathe air, inherited from their lungfish-like ancestors. Life seemed perfectly poised to crawl onto land for good.
But that’s where the fossils disappear. For the first 15 million years of the Carboniferous – between 360 and 345 million years ago – the descendants of the first tetrapods seemed incredibly scarce. Where palaeontologists had expected an explosion of tetrapod species, there were fewer than they could count on their fingers. When the fossil record picks up again, there is a riot of amphibian life at the water’s edge, including the first creatures to fully abandon the water for life on land.
This vast hole in the fossil record is known as Romer’s Gap – after US palaeontologist Alfred Romer, who first called attention to it – and it has long been a mystery. What happened in the period between Ichthyostega pushing its way through weed-choked swamps and the early amphibians making themselves comfortable further ashore? And why so few fossils?
Palaeontologist at the University of Washington in Seattle and others thought the answer might be oxygen. They noticed that for tens of millions of years prior to Romer’s Gap, arthropods like the early arachnids had thrived on land, only to struggle during those mysterious millennia. This led people to argue that the hiatus in the fossil record was down to a severe drop in atmospheric oxygen levels. With less O2 in the air, breathing would have been difficult, creating a barrier to animals taking anything more than brief forays out of the water.
But the fossil record has a habit of surprising us. The first hints came in 2002, when at the University of Cambridge and her colleagues looked more closely at the bones of Pederpes finneyae, previously identified as a fish that lived just within Romer’s Gap, about 348 million years ago. Clack identified Pederpes as a tetrapod with five functional toes on feet that were much less paddle-like than those of its predecessors. It was not fully terrestrial, she says, but its limb anatomy suggested Pederpes was more capable on land than any previous vertebrate. More than that, it was a hint that Romer’s Gap might not be so barren, after all.
“Bigger eyes and better eyesight revealed the bounty of prey on land”
Encouraged, palaeontologists returned to long-neglected rocks dating from Romer’s Gap for a closer look. And they were shocked by what they found. In 2015, Jason Anderson at the University of Calgary, Canada, and others drew from a collection of fossils found at Blue Beach, Nova Scotia, to argue that a had lived there at the time.
The following year, Clack and a team of colleagues returned to a riverbed not far from Chirnside to discover a of new fossils – not one but five new tetrapod species.
The reason they were missed for so long is that the rocks from Romer’s Gap do not contain commercially exploitable resources such as coal, limestone or iron ore. “[The rocks] were considered barren and that meant no one looked,” says Clack. “Simple as that.”
Closer inspection revealed hidden treasures. Among them was Aytonerpeton microps, given the name Tiny thanks to its diminutive head, just 5 centimetres in length. That really is small compared with the other amphibious creatures of the time, and it is part of what makes Tiny so special: it suggests that tetrapods had evolved a range of body sizes by this point. Indeed, although Tiny and its ilk are unlikely to become household names, they have been lauded by palaeontologists because they close Romer’s Gap.
Fossilised charcoal from the same period revealed the presence of fire, giving the lie to the idea that oxygen levels had dropped severely. In circuitous fossil trackways, too, there were clues that invertebrates were not suffering as badly as researchers had previously assumed. It wasn’t that the environment during Romer’s Gap was hostile to life moving ashore: palaeontologists just hadn’t found the fossils.
With the latest discoveries, we’re getting a clear view of this long-standing evolutionary blind spot. Far from suffering in a repressive atmosphere, these early limbed vertebrates were thriving and diversifying rapidly. “This was when tetrapods really began to exploit the land,” Clack says. “The conditions of varying environments seem to have promoted the ability to feed and shelter on land, while still living primarily in the water.”
But why? It is one thing to know that vertebrates were shuffling back and forth between water and land, trialling various anatomical innovations; it’s quite another to know what drove them. Maybe it was the threat of predators in the water, or the lure of new food sources on land?
Malcolm MacIver, a neuroscientist at Northwestern University in Illinois, brings fresh perspective to bear on such questions. He had previously studied how black ghost knifefish detect prey in the water using electrical current, and got to thinking about how the first terrestrial vertebrates perceived their environment. It led him to look deep into their eyes – or at least into the empty spaces of their fossilised eye sockets.
Visions of plenty
MacIver and his colleagues measured the eye sockets of 59 tetrapod skulls from between 390 and 260 million years ago. They found that eye sockets grew dramatically over that period, tripling in size, and gradually migrated to the top of the head. Take Tiktaalik, for example (see image). This was still a fishy, water-bound animal, but the size and position of its eyes suggest it at least paid attention to what was going on above the surface. What surprised MacIver the most, though, was that these changes occurred before the early tetrapods acquired limbs capable of carrying them onto land.
In biological terms, eyes aren’t cheap. It takes a great deal of resources to grow and maintain them. So what benefits did they bring? To find out, MacIver and his colleagues used experimental data from living creatures to model what these prehistoric eyes could have seen, both above and below the waterline. Enlarged eyes aren’t necessarily helpful underwater, they found. But once those bigger peepers poke above the surface, it’s a different story: in air, larger eyes lead to a proportionate increase in how far you can see.
And what was there to look at? Lots of crunchy arthropods, all naive to the threat of predators like these. So here’s what MacIver is suggesting: bigger eyes and better eyesight created “an informational zip line to the bounty of invertebrate prey on land”, perhaps drawing those early tetrapods farther and farther from the water to snaffle up the arthropod buffet. He calls it the “buena vista” hypothesis.
Merely seeing farther was not enough to drive the surf-to-turf transition, of course. But MacIver thinks that once the tetrapod ancestors could catch a glimpse of all that helpless food, those with certain anatomical quirks – like limbs and ribs capable of supporting bodies on land – would have had an advantage that natural selection favoured.
Anderson welcomes the hypothesis, and the creative approach that spawned it, even if he has reservations about how closely eye socket size correlates with eye size. “It’s definitely an idea to follow up on,” he says.
Such is the nature of palaeontology. Every discovery and novel hypothesis raises more questions – and there is still a great deal to learn, not only about Romer’s Gap but also the first slippery steps onto land it has obscured for so long.
At the top of Clack’s wish list is more detail on the reproductive strategies of the creatures making the first forays into a drier world. Were their breeding habits still tied to the water, like those of modern amphibians? When did the first egg capable of surviving on land evolve? “You’d need an [exceptional fossil] showing soft body preservation to stand a chance of that,” she says. Likewise, we still don’t know why five fingers became the standard for tetrapods.
Having exposed Romer’s Gap as nothing more than a sampling problem, we know the answers are out there in the fossil record, says Anderson. Just a few more tiny discoveries could mean a giant leap in our understanding.
This article appeared in print under the headline “The eyes have it”
