Dinosaurs footprints on the Isle of Skye, Scotland
Mark Wilkinson (UK)
If you think of dinosaur hunting, you probably imagine trekking through a parched landscape, reaching the crest of a low hill and catching the first glimpse of a complete skeleton lying half exposed in the next depression. While this might just be true in some parts of the world, the reality of hunting for dinosaurs in Scotland could not be much more different. Hence, a cold and damp day in April 2015 found a small group of geologists from the University of Edinburgh on a slippery foreshore on the northwest extremity of the Isle of Skye.
We were hoping not for complete skeletons but, if we were lucky, an occasional bone or tooth – well, perhaps we were hoping, but plenty of geologists have been here before, so the chances of a large find seemed pretty slim. Having said that, the total number of dinosaur bones that have been found in Scotland is still small, so that any bone is likely to be of interest – and could well be a new species, or evidence that a larger taxonomic group known from elsewhere was present on the island in the Jurassic. To add extra scientific interest, the exposures on Skye include a thick Middle Jurassic sequence, representing a time of a rapid dinosaur evolution, but with a poor fossil record worldwide. So any find might be of great importance.
We visited several locations on the excursion. There are well-known dinosaur footprints at Staffin Bay on the east coast of Trotternish (the northern-most of Skye’s peninsulas), though they can be tricky to locate, and I’ve seen disappointed tourists leaving the beach who have failed to spot them. Unfortunately, only one footprint was well exposed due to the growth of seaweed and a covering of sand. This print has three clear toes (Fig. 1) and was formed by a theropod dinosaur.
We also visited some of the smaller exposures of Jurassic sediments at the north end of the island, but these are typically rather baked by intruded igneous rocks that were formed during the opening of the North Atlantic some, 50myra, and are not a great place for fossil hunting. However, the largest of the exposures at the north end of the island is at a beach known as Cairidh Ghlumaig, which we usually refer to as Duntulm (since most of us do not speak Gaelic), after a castle a few hundred metres to the north, which is placed dramatically on top of an inaccessible cliff made of layered dolerite and which is a Site of Special Scientific Interest (SSSI; Fig. 2).
The exposure of sediments is not so dramatic, just a wave-cut platform extending perhaps 500m along the beach, and is entirely covered by the sea at high tide. At least it is accessible – only a few tens of metres from the road and a short grassy scramble down to the boulder beach.
One of our first finds was a crushed bone fragment approximately 4cm in length (Fig. 3), which is set in a block of white sandstone.
This caused a fair bit of excitement – if we could identify the bed that this was from, we could search that bed and hopefully find more. Even finding more blocks of the same lithology would give us some hope of further finds. An extensive search of the beach failed to locate even a single pebble of the distinctive white sandstone, or the bed in situ. We still have no idea where this block (which is about 30cm across) came from or its age.
This is a problem in Scotland, where, during the last glaciation, large blocks of rock were transported sometimes long distances by glaciers. The fact that the bone is crushed (which is unusual here) lead to some speculation that it was originally hollow, hence light in weight and might have belonged to something that flew or perhaps could glide. At present, this remains only speculation.
We found a selection of small fossils, mostly shark’s teeth in the 1 to 5mm size range, and as the tide came in, we were ready to give in and go back to our accommodation for a well-earned meal. As we walked back across the exposure, our two most experienced palaeontologists (Tom and Steve) spotted something odd – a series of circular depressions in one of the sandstone beds (Fig. 4). The depressions are generally approximately 50cm across and unevenly spread across the surface.
At first glance, they don’t look like anything special – and no geologist has previously thought them worthy of description, although plenty have visited the site, which features in geological guide books to the area. However, some of the features showed four projections, and were clearly footprints – not of theropods as with the Staffin examples, but from sauropod dinosaurs. We quickly realised that we had discovered a bedding plane covered in footprints, with several identifiable trackways (that is, lines of prints made by a single animal as it walked along).
Many of the prints are preserved simply as shallow depressions, but others stand up compared to the surrounding rock, which has been preferentially eroded away. A fair number of dinosaur footprints have been previously found on Skye, though most are in loose blocks – some can be seen in Staffin Museum at Ellishadder on the east coast of Trotternish and a visit is recommended if you are in the area. One footprint at Duntlum was helpfully preserved as part of a loose block, which gave us a cross-section through it.
Fig. 5 shows Tom and Steve taking photos, which were later built into a 3D digital model using computer software.
The tracks were made by sauropod dinosaurs, which included the largest land animals that have ever lived. They originated in the Triassic, but were an important part of the Jurassic fauna. They had five toes, and the name sauropod reflects a similarity to the feet of modern lizards. They also had very long necks and tails, comparatively small heads, and four thick legs to hold the weight of the body. The most famous species might be Brontosaurus, although palaeontologists seem recently to be unable to agree if the name is valid. Although some sauropods were huge, more than 20m in length, they were all plant eaters.
Before finding the footprints, we had only limited evidence that this important group of dinosaurs were present in Scotland – a few isolated bones and teeth were all that was known. However, footprints cannot be moved by rivers unlike a dinosaur carcase – so the animals were living just here, for sure. Unfortunately, the chances of identifying which species made the footprints is slim, as trace fossils (of which footprints are only one rather uncommon type) are great for studying animal behaviour, but similar traces can be made by multiple similar species. Interestingly, there are only a small number of sauropod trackways known in the world from the Middle Jurassic, including important sites in England, Mexico, the USA, Morocco and Portugal, making our quite important.
So, what was the Isle of Skye like when dinosaurs roamed the land? We know that the sediments that they left their prints in were in or close to the sea, as there are numerous marine bivalves as well as the sharks’ teeth – though no truly marine fossils such as ammonites. Bivalves are found in great abundance in some beds, which may represent banks of shells left by storms. However, the sediments show no sign of the currents that might be expected in open water, which would leave cross-beds, and there are well-preserved algal limestones that were most probably growing in sheltered conditions. Equally, there are no desiccation cracks – these cracks, familiar to anyone who has looked at the bed of a dried-up lake or reservoir, are a sign that a sediment has been exposed to the air and dried out. So the dinosaurs were probably walking in shallow lagoons with banks of shells, close to the sea, but not in the open sea.
As sauropods were plant eaters, there must have been plenty of vegetation around, though perhaps not here precisely, as we have found no evidence of tree roots. Fossil logs from trees are common at other locations on Skye from sediments of similar ages, so the land must have been at least partly forested. As for the climate, it was sub-tropical, unlike our chilly visit in April. The day we left to drive back to Edinburgh, the island was bathed in glorious spring sunshine…
The story of the discovery was released to the press in December 2015, after the scientific paper describing the find was published in the Scottish Journal of Geology. The story was reported in many UK newspapers, and technical journals such as Science magazine. On the television, STV made a report in April (http://news.stv.tv/highlands-islands/318914-edinburgh-university-dinosaur-hunters-make-fresh-discoveries-on-skye/), while the BBC aired the story on the 6 o’clock news, BBC Landward and BBC Newsround. The BBC also made a superb website that included the find (http://www.bbc.com/earth/bespoke/story/journey-to-jurassic-island/index.html), along with more information about the dinosaurs of Skye.
What of the future for the site? We need to map the footprints accurately, perhaps using a drone-mounted camera. This works well provided that there isn’t too much wind – and Skye is a windy place. We are not yet sure how many animals are recorded here, or if they are all adult or include some juveniles. As the tracks are cross-cutting, this might not be easy to work out for sure. Indeed, the Skye footprint site has been nicknamed the ‘dinosaur disco’ from the confusion of prints. At least we have an excuse to revisit the island, which, on a fine day, is outstandingly scenic and well worth a visit.
Dinosaurs of the British Isles, by Dean R Lomax and Nobumichi Tamura, Siri Scientific Press, Manchester (2014), 414 pages (softback), ISBN: 978-0-9574530-5-0
Scottish Fossils, by Nigel H Trewin, Brocken Dunedin Academic Press Ltd, Edinburgh (2013), 118 pages (hardback), ISBN: 978-1-780460-019-2