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Footprints discovered over 40 years ago in rocks from Yorkshire have been revealed as potentially the earliest prints from an amphibian yet discovered in the UK, and are potentially the oldest from any four-legged animal.
Made over 340 million years ago during the Carboniferous Period, they help us understand how the continents have shifted over time.
A new analysis involving a 3D model of the fossil footprints discovered in the 1970s at Hardraw Force waterfall in Wensleydale has shown that they are likely the earliest tracks of amphibians ever found in the UK.
The slab of rock, just 50 centimetres across, has been in the Museum's collection for decades. While it has long been suspected that they were of great interest, their significance has only now been confirmed.
The work was carried out by Hannah Bird from the University of Birmingham in collaboration with Dr Angela Milner, a Scientific Associate at the Museum, in collaboration with colleagues at the University of Birmingham and University of Cambridge.
'This particular specimen is really nicely preserved, but it has only been briefly mentioned in literature before and it seemed a shame that nothing substantial had been published on it,' says Hannah. For a previous research project, Hannah had identified it as potentially one of the oldest set of footprints in the UK.
'It was a matter of validating whether this was the case or not, including comparing them to other footprints from other localities across the world to evaluate timescales,' explains Hannah.
'From that we concluded that they were the oldest footprints of this type. We can't necessarily say that they are definitely the oldest footprints, as previously specimens were collected with less detailed information on the rocks they were sourced from, but we can say it is the oldest known that we have managed to find.'
The tracks are thought to have been made when an ancient relative of modern amphibians known as an edopoid temnospondyl walked over some soft sediment of a river delta. These would have looked something like a large salamander, with four digits on the front foot and five on the back.
'You can tell when looking at tetrapod footprints, whether they were made by reptiles or amphibians,' explains Hannah. 'Reptiles have long, slender digits, whereas amphibians generally have shorter, broader digits. So automatically you can tell when looking at this specimen that it has these short, stubby digits that are amphibian.'
Temnospondyls were highly successful animals, appearing during the Carboniferous and evolving over the next 210 million years into a wide variety of forms. While some were about the same size as modern-day amphibians many grew to enormous sizes.
'Some of these things were large crocodile-like animals at least two metres long with big heads,' explains Angela. 'They have been nicknamed by some of my colleagues as croco-manders, because they look like a crocodile but had the same lifestyle and walked the same way as salamanders.'
Most temnospondyls were semiaquatic, filling a similar role in the swamps of the Carboniferous, Permian and Triassic as crocodiles do today. Despite being some of the first truly large terrestrial tetrapods, all had to return to the water in order to breed.
If you were to travel back to Wensleydale 340 million years ago, you would find that the rolling hills of the Yorkshire Dales would be replaced by a vast river delta teeming with life.
Long before modern trees and plants evolved, the land would have been dominated by huge horsetails, club mosses, scale trees and ferns. It was a Period during which invertebrates managed to grow to enormous sizes, as amphibians dominated the predominately swampy environment.
'One of the closest modern analogies would be the Mississippi delta in North America,' says Angela. 'The animals would have been living in the river delta, and as the water receded and then flooded again there would be all sorts of areas that were exposed that these freshwater animals were walking over.'
The trackway is not a new find. It was first discovered in 1967 by John Chapman as he walked behind the waterfall Hardraw Force. Located in Wensleydale in the north Yorkshire Dales, it is the longest single-drop waterfall in the UK, falling 30 metres from a rocky overhang.
Around the bottom of the waterfall, the ground is littered with sandstone slabs that have fallen out of the cliff. It was on one of these slabs that Chapman spotted a handful of preserved prints which he donated to the Museum in 1978.
'I thought that the fossil was interesting from the outset,' says Angela, who first identified the specimen at the Museum.
'Now we've discovered what kind of animal the footprints actually belong to it has made the fossil even more significant as there were no footprints of these animals of that age in the UK. We haven't found anything like this in Britain before.'
While it is impossible to say that these are definitively the oldest fossil footprints in the UK, and there may be some fossil footprints of around a similar age in Scotland, they are the best preserved and oldest amphibian fossil footprints found to date.
The footprints are on display in the From the Beginning gallery among other fossils and specimens that represent how life on Earth evolved.