January 30 , 2017:
by Elsa Panciroli
I held in my hands one of the first animals to drag itself out of the water and onto land. Christened “Tiny”, she (I like to think it was female, but we can’t actually tell) is entombed in a chunk of boring, black rock. I ran my hands over the surface, feeling the bumps betraying a stone pregnant with fossils. Yet, from the outside there’s no sign of the treasure within: no one has ever actually seen it.
“We didn’t really know it was in the small piece of rock that we collected until it was CT scanned,” Dr Nick Fraser, Keeper of Natural Sciences at the National Museum of Scotland in Edinburgh told me. “We were quite surprised to find Tiny hiding in the sediment - we still only know it from the 3D scan and the 3D print and so haven’t had the pleasure of seeing the actual fossil!”
Comfortably fitting in your hand and looking superficially reptile-like, Tiny is a rare fossil indeed. She comes from a time in the rock record that has traditionally yielded very few fossils. This time was named Romer’s Gap, after the scientist who identified the paucity, and it has achieved palaeontological notoriety both as an intriguing enigma, and because this enigma obscures the development of the first animals to live on land. What caused the gap? Mass extinction? Low oxygen? Or is there no gap at all, are we just not looking hard enough? Recent finds from Southern Scotland (including Tiny) suggest the latter may be true, yielding exciting new fossils of the earliest land-living vertebrates.
Tiny and I have a lot in common. We both have backbones, hinged jaws, lungs and four limbs. All of these are important steps in evolutionary development leading to the group called tetrapods, literally ‘four-footed’ in Greek. Tetrapods include amphibians, mammals, reptiles and birds.
Tiny and I both grew up on the shores of a Scottish loch; but whereas mine was a deep, cold, freshwater one, hers was tropical and probably brackish. During her lifetime, 350 million years ago, Scotland was part of an ancient landmass called Euramerica, straddling the equator. It was hot, around 30 Celsius, and lush green with ancient lycopod forests. A paradise you may be thinking, but watch out for the copious scorpions and centipedes crawling through the undergrowth.
The most important thing that Tiny and I may share however, is having five digits at the end of our limbs. If Tiny had this, she would be among the first animals who could play This Little Piggy.
“There are two [animals with five digits] that we know for certain: Pederpes, and an isolated foot found by our project,” Professor Jenny Clack, Emeritus Professor at the University of Cambridge, explained the evolution of limbs and digits to me. Her latest multi-authored scientific paper included the jumble of fossil bones that was Tiny. Many of the first tetrapods had more than five fingers and toes on each limb, some as many as eight. Clack elaborated “as limbs with digits evolved from fins, there must have been experimental forms, and the genetic regulation was less precise, allowing variability. These fish-like animals did not ‘walk’ in the conventional sense.”
Professor Clack is one of the biggest names in palaeontology. An unassuming woman in her late 60s, she has spent a lifetime researching the earliest tetrapod fossils in order to understand the evolution of the first land vertebrates – often called the “fish to tetrapod transition”. In the 1980s Clack and her team of researchers recovered fossils of Acanthostega and Ichthyostega from rocks in Greenland. These were some of the animals undergoing this “transition”, and their anatomy surprised scientists. It seemed limbs had evolved from fins not for dragging the animal from shallow pool to pool, as previously suggested, but for movement within water. The first limbs allowed our ancestors to push their way through dense underwater vegetation in their swampy habitat.
But why, I asked her, did five fingers turn out to be better than eight? “We’ll probably never know for sure,” Clack answered, “but I reckon it’s to do with producing a wrist or ankle that is both weight-bearing and flexible. The multi-digited forms did not have this; the limbs were stiff and paddle-like without good joints.”
Clack is one of the central driving forces behind the TW:eed project (Tetrapod World: early evolution and diversification), which includes researchers from the National Museum of Scotland, Universities of Leicester, Cambridge and Southampton, and the British Geological Survey. These researchers focus their considerable expertise on the rocks of Southern Scotland. These rocks are younger than those in Greenland, being from the Tournaisian (360-345 million years ago) - the notorious Romer’s Gap. This period marks the end of the Devonian, often referred to as the “age of fish”, and the beginning of the Carboniferous.
TW:eed have now successfully bored and logged two cores through hundreds of metres of solid Tournaisian rock. (If you’re unfamiliar with geological “logging”, think less lumberjack and more sedimentary accountant: it is the painstaking, detailed inventory of layers of rock in a sequence, telling you how the environment changed over time.) TW:eed’s logging in Scotland has been done to an impressive centimetre scale, making these the most detailed accounts of Tournaisian sediments in the world.
“It does appear that if there had been a ‘gap’ it was much smaller than previously thought, and might have affected some groups less severely than others,” Clack told me, talking about the disappearance of many species at the end of the Devonian. “There was an extinction event for many fish species, but no-one is really sure what caused it.”
Clack and her co-authors found evidence in their rock cores that fires burned throughout the Tournaisian, challenging previous theories that low atmospheric oxygen during the time period caused extinctions. “Climate change, possibly, but not certainly. Whatever it was affected the flora as well, eliminating the large lycopsid trees that reappeared only five or six million years later.”
Tiny was not alone in her scorpion-infested Scottish swamp. Only one species, Pederpes finneyae, was previously named from this time period, but Clack and colleagues have named five new species, and found many more fossils too fragmentary to formally identify. This suggests that there may be a chance of finding further important Tournaisian fossils in Scotland.
“There is every chance!” Clack enthused, “no-one bothered to look at the right rocks previously. Because they don’t yield commercially exploitable resources these rocks did not have a history of accidental finds. So because no-one had ever found anything, no-one ever looked, and they were considered barren. It was a vicious circle.”
So what about Tiny? I visited Nick Fraser’s office at the National Museum of Scotland. On his desk between a cast of ancient reptile footprints and a battered taxidermy fish, lay a small chunk of white plastic, bearing nippy teeth and a chin full of dimples. It was a printed model of Tiny.
“We estimate that Tiny had a skull that was about five centimetres long. In other words pretty tiny, and certainly so when compared with some of the other tetrapods we’ve been finding,” Nick handed me the model, which had been enlarged to make the features clearer to the naked eye.
Thanks to micro CT-scanning, we needn’t break open the protective casing of rock in which Tiny was found: we can digitally reconstruct this creature hiding inside, in all her miniscule, toothed glory. Her scientific name, Aytonerpeton microps, delightfully translates as ‘the creeping one from Ayton with the small face’ (Ayton is the Scottish parish in which she was found). Her sharp little teeth probably crunched up those Euramerican scorpions; indeed the copious invertebrates that conquered life on land 60 million earlier would have provided a tempting menu for tetrapods venturing from the water.
I asked Fraser if Tiny had five fingers and toes: “We can’t be sure about the number of digits as it is somewhat disarticulated,” he replied, “but what remains of the toes don’t hint at more than 5 digits.”
These fossils may not be as sexy for the public imagination as dinosaurs, but they are arguably more important. They chart the way in which our ancestors’ skeletons adapted to life on land and the timing of our ancestors’ split from the ancestors of amphibians. As we find more fossils, we learn about the diversity of life on earth over 345 million years ago, and that Romer’s Gap may not be so much a gap, as a blind-spot.
I suspect that over the coming years we shall continue to find important additional fossils in Scotland
Dr Nick Fraser, National Museums Scotland
“Currently Scotland has by far the richest assemblages of fossil tetrapods from the Tournasian anywhere in the world. So presently it is the most important place for trying to understand the nature of the early invasion of land by animals with backbones,” Fraser told me. “And of course this is a pivotal step in the evolution of life on land - without it there would have been no salamanders, no frogs, no crocodiles, no lizards, no dinosaurs, no birds, no mammals and therefore of course no humans. So pretty critical!”
When asked if Scotland was the only place to find such amazing fossils, Professor Clack replied “we are sure the right rocks exist elsewhere, should anyone be motivated to look - which we hope they now will be.”
If you want to find out more about Scotland’s amazing Romer’s Gap fossils, the Fossil Hunters exhibition is now on tour around Scotland. It has already open in Biggar (running until the 15th January), soon to reach Montrose (21st January to 1st April), and finally Stornoway.
Clack JA, Bennet CE, Carpenter DK, Davies SJ, Fraser NC, Kearsey TI, Marshall JEA, Millward D, Otoo BKA, Reeves EJ, Ross AJ, Ruta M, Smithson KZ, Smithson TR, Walsh SA. 2016 Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature Ecology & Evolution, 1, doi:10.1038/s41559-016-0002.