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Carboniferous Yorkshire: the oldest fossilised amphibian trackways in the UK

Samuel Lickiss, Editor at the Geological Society’s Publishing House, speaks to two authors of a paper published today in the Journal of the Geological Society that investigates the UK’s oldest fossilised amphibian trackways that have been held at the Natural History Museum for nearly 40 years.

Fossilised amphibian trackwayIt feels like a long time ago now, but my first proper field trip as a geology undergraduate was to Ingleton, North Yorkshire, mapping the beautiful sequences of rocks along the waterfall trail. Given that scores of geologists over the years have intimately studied this stunning landscape, it may be surprising to find out that there’s still plenty more to discover.

Early Carboniferous Yorkshire, about 340 million years ago, was a very different place. ‘The tracks are preserved in a micaceous sandstone unit,’ says Hannah Bird, lead author of a study published this week in the Journal of the Geological Society. ‘This represents a river delta system entering a shallow epicontinental (inland) sea.’

Back in the Visean stage of the Carboniferous, the UK was positioned much closer to the equator, resulting in ancient Yorkshire being covered with tropical forests - perhaps not too dissimilar to the swampy areas around the modern Amazon basin or the Mississippi delta system. ‘These areas of shallow water don’t experience high energy input, allowing for an assortment of invertebrates to thrive in the area, as seen by the good preservation of numerous invertebrate traces alongside the footprints.’

The fossilised tracks were found in a limestone gorge carved out by Hardraw Force, England’s highest waterfall, in 1978. Angela Milner, co-author of the paper, first described the trackways when they arrived at the Natural History Museum: ‘The tracks were a serendipitous discovery. If the slab had landed the other way up after it fell out of the cliff next to the waterfall, it would never have been recognised and collected.’

I asked Dr Milner to describe her first encounter with the fossils: ‘I realised that it was a track of an early temnospondyl amphibian, so obviously an important record. I looked through the available literature to identify the footprints and Megapezia was the closest match based on outline drawings dating back to 1903 and a very poor 1970s photograph of a trackway.’

Just as organisms have their binomial Latin names, trace fossils - the artefacts left behind by living things like footprints, burrows and coprolites (fossilised faeces) - have a similar taxonomic system. Megapezia is a one such ichnogenus, trackways left behind by temnospondyls that Dr Milner describes as, ‘a crocodile-like amphibian that may have looked something like a giant salamander’.

Temnospondyls are particularly important as they are thought to be among the first creatures to crawl out of the oceans and begin colonising land, and for this reason, Dr Milner included them in the NHM’s From the Beginning gallery, documenting the ascent of life. They’ve been on public display since.

So why are these trackways being studied again after so long?

Hannah Bird, who began the research while still an undergraduate, explains: ‘The project developed from my integrated master’s project whilst studying at the University of Birmingham, which was developing the first comprehensive database of tetrapod fossil footprints within the UK, using literature searches and museum collections. The specimen studied in the paper was one of those included, and it piqued my interest as it had only briefly been studied by Angela and since neglected.’

‘I was only too happy for her to include it and the newly published paper is the result,’ says Dr Milner. The tracks were 3D scanned revealing details not previously visible, which led to them being reclassified from Megapezia to Palaeosauropus, which, as both Ms Bird and Dr Milner point out, are synonymous.

More interestingly, however, the team were able to figure out which direction the temnospondyl was moving. One of the challenges in all branches of geology is determining direction of movement. Whether it be a lava flow, landslide or a fossilised trackway, figuring out how A got to B is an important process in reconstructing the palaeoenvironment. Often, that’s easier said than done.

‘The intuitive trackway you see when looking at the specimen, with a gently curving midline, was the conclusion drawn during writing of my master’s dissertation,’ says Bird. ‘However, with further study, we re-evaluated the footprints through greater analysis from being able to see more detail in the 3D scans.’

With several direction hypotheses in tow, Bird took an interactive approach to interpreting the movement of the organism: ‘At one point I was actually trying to replicate the movement and hand/foot positions being suggested by going on my hands and knees on the floor – it was like a game of Twister and proved its unsuitability as a possible movement! It seemed to be one of these things that the more we thought about it, the more confusing it became.’

Figuring out whether an individual imprint represented a hand or a foot proved a challenge. ‘There are distinct difficulties when making classifications as numerous environmental factors affect the footprint morphology at time of formation,’ says Ms Bird. ‘These include moistness of sediment being sufficient to maintain the shape of the footprint, clay content of wet sediment creating a sucker effect when the limb is lifted, alongside wind and water erosion removing the footprint before it can be preserved.’

Nevertheless, the 3D scans allowed the team to untangle the temnospondyl’s movement direction and infer some clues about its lifestyle. ‘It likely had a semi-terrestrial mode of life, meaning that it walked and fed on land, but used water bodies for breeding. This therefore differs somewhat from modern amphibians, which rely more heavily on a moist environment to survive.’

These footprints, the oldest-known amphibian tracks in the UK, demonstrate that there’s still much to learn about the evolutionary history of the world. ‘In the future, we may even make discoveries of footprints older than this one, helping to extend the tetrapod record of Britain further into geological history.’

‘I feel really excited that the paper lead by Hannah has added so much knowledge to the history of early amphibians and their palaeoenvironment in the UK,’ says Dr Milner, but she tells me she has no further plans to study footprints: ‘I shall be getting back to specimens with skeletons!’

While Dr Milner may be returning to skeletons, Hannah Bird wants to focus more on palaeoclimatology: ‘I wish to translate knowledge into concepts with applicability to the modern day, especially with the current focus on climate change. Therefore, I do intend to continue with PhD research which encompasses these avenues, furthering both academic research and science outreach.’

Their discovery might be serendipitous, but the techniques used to unravel their geological significance are not, and Dr Milner and Ms Bird tell me there’s still plenty more to find out there. I for one look forward to seeing what else we can learn using modern 3D scanning techniques when combined with traditional geological know-how and, perhaps, a few more rounds of palaeontology Twister.