Last month I published a non-technical summary of the scientific paper describing Scotland's new dinosaur discovery, StEiggosaurus. This month I'd like to do the same for another of my papers: Diverse vertebrate assemblage of the Kilmaluag Formation (Bathonian, Middle Jurassic) of Skye, Scotland, which came out online at the end of July 2020. It summarises discoveries made to date from part of the rock series on the Isle of Skye - the Kilmaluag Formation, one of the most fossiliferous sites in the UK for vertebrate animals - and compares this with other sites around the world.
Below is a non-technical summary of the paper. No matter your background you can find out about the scientific discoveries being made in Scotland!
Introduction
The Middle Jurassic (164-174 million years ago) is a pivotal time for many land-living animal groups. Not only is there an expansion in the number and kinds of animals (including dinosaurs), but we can also trace the origins of groups that are still alive today. These include mammals, squamates (lizards and snakes) and amphibians. To understand how these groups emerged, and the evolutionary changes that led to their success and diversity in later time periods, we need to study their fossils.
Looking around the world, the fossils of Middle Jurassic land animals are quite rare compared to other parts of the Jurassic, and the Cretaceous which followed. Some of the best-known examples come from China, Russia, and the British Isles. In the UK, the fossils from sites in Oxfordshire and Gloucestershire in England have been among the most studied and celebrated to date. They have been especially rich in very small backboned animals, the exact groups that scientists are so keen to understand because of their long-standing role in Earth's ecosystems.
There is a series of rocks on the Isle of Skye collectively called the Great Estuarine Group. It is one of the most fossiliferous rock layers in the UK, and as such is legally protected as a SSSI (Site of Special Scientific Interest) and through Scotland's new NCO (Nature Conservancy Order). All work on these rocks requires a permit, and fossils can only be collected for scientific purposes. Work has been carried out on the fossils in these rocks in the Southern part of Skye since they were first discovered in 1971 by Michael Waldman. Since that time many different researchers have worked there, notably Robert Savage (University of Bristol), Susan Evans (University College London) and Paul Barret (Natural History Museum, London). In the last decade Stig Walsh (National Museums Scotland), Roger Benson (University of Oxford), Richard Butler (University of Birmingham) and I have been the main team working on the Kilmaluag Formation.
Our research shows that the fossils from this layer of rocks are globally significant in terms of their completeness and preservation. This makes them vitally important for ongoing studies into the origins of multiple animals groups, and places these Scottish rocks among the most important Middle Jurassic localities in the world.
Geology of the Kilmaluag Formation
The Kilmaluag Formation is one of seven rock formations in the Great Estuarine Group, found in the Inner Hebrides of Scotland. It dates to the Bathonian, around 166 million years ago. This series of rocks captures a changing landscape as the area lifted and fell due to geological processes, causing the land to dip above and below sea level. This periodically created shallow seas, or more terrestrial lagoon environments and deltas. Much later in the Palaeogene around 55 million years ago, these rocks were covered by volcanic eruptions, which also cut through and heated the rock in places. Now the Great Estuarine Group is only visible where the volcanic rock has been eroded by water and ice to reveal the older rocks underneath. This happens predominantly along the shoreline.
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The Great Estuarine Group, which includes the Kilmaluag Formation (circled in red).
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The Kilmaluag Formation is found on the Isles of Skye, Eigg and Muck, and is up to 25 metres thick. It's named for the village of Kilmaluag on the north coast of Skye, but although you can find some of these rocks there, the largest exposed parts of the Kilmaluag Formation are on the Strathaird Peninsula in the South of the Island. The rocks are Late Bathonian in age, and they are unique because unlike the rest of the Great Estuarine Group, they represent an almost entirely freshwater environment. We know this from the numbers of freshwater organisms preserved in them, including small crustaceans called ostracods (the Kilmaluag Formation used to be called the Ostracod Limestone).
In the north of Skye, the Kilmaluag Formation mostly comprises sandstones, whereas in the south of Skye and on Eigg and Muck, it is mostly limestone. There are cracks running through some of the beds, showing us that there were periods when the sediments dried out completely and cracked, before being filled-in during wetter climate cycles. The presence of muds with lots of clay and carbonates tells us this was once a landscape covered in freshwater lagoons. The layers that yeild the most vertebrate fossils were probably just above the shoreline, and dried out from time to time before being flooded again.
Fossil Plants and Animals from the Kilmaluag Formation
Plants
There hasn't been much work on the plant fossils from the Kilmalag Formation. There are some small bits of bark and stem visible in the rock, but they are very broken up. A study carried out in 1991 looking at the small spores left by plants in the fossil record from Skye, found pollen in the rocks on the north of the island. The pollen mostly came from plants like conifers, cycads (gymnosperms) and ferns (pteridophytes).
Invertebrates
There are lots of invertebrate animals in the Kilmaluag, mostly in the form of ostracods (Darwinula and Theriosynoecum), conchostracans (Anthronestria and Pseudograpta), gastropods (Viviparus) and bivalves (Unio). There are some trace fossil burrows that are thought ot have been made by some kind of crab or shrimp.
I discovered the first insect fossils from the Kilmaluag in the north of island in 2017 - mostly beetle wing cases - and more have been found since then. They're currently being studied by researchers at National Museums Scotland in Edinburgh, and the Huntarian in Glasgow.
Fish
A lot of fish fossils are found in the Kilmaluag Formation. Fossil sharks, Hybodus and Acrodus are commonly represented by their teeth, and they are some of the only non-marine Jurassic Acrodus fossils in Europe, and youngest in the world. The scales of other kinds of fish have also been found, and there are also some skeletons which are now being studied.
Amphibians
There are two species of salamander known from the Kilmaluag Formation: one called Marmorerpeton, and the other known only as 'Kirtlington salamander A'. Both of these were previously found at the English fossil locality called Kirtlington Cement Quarry, in Oxfordshire. Marmorerpeton is a relatively large (around 30 cm in length) aquatic salamander. It was previously only known from just a few scattered pieces of bone, but there are now several partial skeletons from Skye, which are being studied by our group and our collaborators. It's significant because it belongs to the very earliest group of salamanders that evolved in the Middle Jurassic, so knowing more about its skeleton will help us understand the emergence of the whole group.
Another amphibian found on Skye is Anoualerpeton. It belongs to a group called the albanerpetontids, which went extinct just 2 million years ago. There are no frogs known from the Kilmaluag Formation at the moment - we'll keep looking!
Lepidosaurs - 'Small Reptiles'
The lepidosaurs are the taxonomic group that include snakes, lizards and the tuatara (found in New Zealand). Although often colloquially referred to as 'small reptiles', some of them can grow very large. Their main uniting trait is that they have overlapping scales on their bodies.
There are lots of early ancestors of lepidosaurs in the Kilmaluag Formation - these fossils are really important for our knowledge of the evolution of this animal group, which is now common throughout the world. Most of these lepidosaurs are known from jaws and some pieces of the skeleton. One of the most common on Skye is Marmoretta, which is now known from lots of jaw bones as well as a partial skeleton - the most complete in the world to date. Other lepidosaurs include Balnealacerta, Bellairsia and Parviraptor. Partial skeletons of these animals are all currently being studied by our group and our collaborators.
Turtles
The Kilmaluag Formation yeilded Scotland's very own Jurassic turtle: Eileanchelys waldmani. It was found in 2004, and represents one of the earliest aquatic turtles in the fossil record, and one of the very few known from this time period around the world. It belongs to near the base of the family tree of living turtles, so it's anatomy helps us understand the evolution of turtles in the time of dinosaurs. Turtle bones are relatively common in the Kilmaluag Formation.
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A selection of fossils from the Kilmaluag Formation (clockwise from top left): the teeth of a shark; the jaw of a mammal; the jaw of a lepidosaur' the toothof a dinosaur; the head of a turtle; the tooth of a close mammal relative.
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Choristoderes
Choristoderes became extinct around 20 million years ago, but they had their roots some time prior to the Middle Jurassic. They were semi-aquatic reptiles, and mostly quite small (hand sized). Their exact relationship to other reptiles is not certain, partly because of a lack of more complete fossil material. The choristodere Cteniogenys is found on Skye, including bits of the skeleton and a small skull. This is the most complete material known for this animal, and is currently being studied to see what it reveals about the group.
Unidentified Small Reptiles
There are at least four unidentified types of reptile in the Kilmaluag Formation, but unfortunately they don't have any of the features needed to place them in their own genus or species. Hopefully future discoveries will make it possible to name them, and tell us more about what group they belong to.
Crocodiles
A partial skeleton was found in the Kilmaluag Formation and described in 1996. It includes parts of the limbs, ribs, vertebrae and osteoderms, the hardened scales that made up the skin. Some teeths and other bits of crocodile have been found, but mostly not figured or described yet.
Pterosaurs - Flying Reptiles
There are two pterosaur skeletons known from the Kilmaluag Formation, both partial. These are the first flying reptiles to be found in Scotland, and are currently under study.
Dinosaurs
Very little dinosaur material has been found in the Kilmaluag Formation so far. A single tooth belonging to a sauropod, a tooth belonging to a theropod, and an incomplete limb bone, are all that is currently known. Footprints have been found in the northern part of the island, including large and small theropod prints which might represent adults and their young.
Mammals and Kin
The first Jurrasic mammal from Scotland was found in the Kilmaluag Formation in 1971, Borealestes serendipitus, along with the first close relative to mammals, an animal called a tritylodontid. Since then many different early mammals have been found on the island.
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A jaw from Borealestes, found a few years ago. Only the tips of the teeth were visible on the surface, the rest was seen and studied by CT scanning the rock.
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Borealestes belongs to a group called the docodontans, which are an early offshot of mammals. They interest scientists because we've dicovered in the last 20 years that despite being from such an early part of the mammal tree, they had evolved very specialised adaptations to their environment - something we thought was exclusive to more recent mammal groups. The partial skeletons of several docodontans are currently under study (a new paper on this should be out very soon!) and should tell us more about their evolution.
Another early-branching mammal is Wareolestes, which is known from Skye from a single jaw. The jaw had replacement teeth just emerging, indicating it was a young animal about to replace it's milk teeth. Other mammals discovered from the Kilmaluag Formation are closer relatives to living groups. They include Palaeoxonodon, which is known from a near-complete jaw found in 2015 (and some other pieces of jaw). The specimen told us that many of the species identified previously from Oxfordshire based on individual teeth, were actually the same animal. The differences in tooth shape were due to the position of the tooth along the tooth row, rather than being fundamental differences between species. This has a knock-on effect for our estimates of how diverse the mammals were in this time period.
My collaborators and I are currently studying some partial skeletons of several species of mammal from the Kilmaluag Formation.
Comparisons With the Rest of the World
Looking at the animals found on Skye and comparing them to other localities globally, we can see that Skye has a really diverse number of vertebrate fossils. Many of the same species are found at Kirtlington Cement Quarry in Oxfordshire, which dates to the same time period in the Middle Jurassic. But the fossils from Skye are generally more complete than those in England, and so provide new information about the anatomy of these creatures. In many cases, the fossil material found on Skye is the most complete example of these species, especially the small reptiles, salamanders and mammals.
Some groups known from England haven't been found yet on Skye, including frogs, the group of lepidomorphs called rhynchocephalians, and haramiyid and multituberculate mammals. It might be that the English site represents a slightly different environment where these animals didn't live, but it could also be that differences in how we collect fossils on Skye (see Collecting below) account for these differences.
Similar animals are found in Middle Jurassic sites in Russia (Itat Formation) and Morocco (Guel el Ahmar Fauna). The similarities with Russia are strongest, except that there are haramiyid and multituberculate mammals in Russia (and not Scotland, yet). The Middle Jurassic of Morocco is still only poorly known, but it is interesting that there could be similarities, because it was separated from Skye in the Middle Jurassic by the growing Atlantic Ocean. Hopefully more material from this and other sites from the Southern continents will provide more information.
The Jurassic fossils of China are certainly the most spectacular examples of their kind in the world. Although much younger than those on Skye (late Jurassic in Age), they include some of the same groups, such as small reptiles, pterosaurs, theropod dinosaurs and the docodont mammals. There is a much higher diversity of dinosaurs in China, but there are more small reptiles in Scotland, and similar numbers of mammal species. Most of the fossils from China are exceptionally complete, but they are preserved and collected quite differently from those in Scotland. Their spectacular appearance can give the impression that China held an ultra-diverse and unusual collection of animals with lots of unique specialisations. However the same types of animals are known from elsewhere, they are simply much less complete athan those in China.
Other Late Jurassic sites include the Morrison Formation (Late Jurassic, USA) and Alcobaça Formation (Guimarota, Portugal). Collecting has taken place in the Morrison Formation for a long time, and the list of animals found from those layers is huge. As in Scotland, choristoderes, fish, salamanders, crocodiles, pterosaurs, dinosaurs and mammals are known from the Morrison Formation, but there are lot more species known than in Scotland. The Alcobaça Formation in Portugal also has some similarities with the Kilmaluag Formation: similar species of fish, small reptiles and mammals are known. Both sites also have crocodiles, pterosaurs and dinosaurs, but in greater numer os species in Portugal to date.
The Purbeck Formation in England is another site worth comparing: it is Late Jurassic to Early Cretaceous, and is very diverse, being one of the most diverse vertebrate fossil Formaton in the UK. Although similar groups of animals are represented there, there are very few crossovers with the Kilmaluag in terms of the actual species - except for some of the fish, and the squamate, Parviraptor.
One interesting thing when comparing the geologically later sites like the Morrison, Alcobaça and Purbeck Formations, with the older Kilmaluag Formation, is that the mammal groups change noticably. In these later Formations there are fewer of the early-branching groups of mammals like docodontans, and more members of more modern groups, including multituberculates and haramiyidans.
Collection Methods and Potential Biases
Of course when comparing the Kilmaluag Formation animals with other sites, we have to bear in mind the methods used to find and extract fossils. These methods effect what palaeontologists find at a site, and so must be taken into account when assessing the fossil assemblage compared to other locations. In the Kilmaluag Formation fossils are found only when they are visible on the rock surface. They are extremely difficult to remove from the site due to the nature of the rock and the location (and they are also only collected when deemed scientifically valuable, to limit our impact on this sensitive area). Once extracted, they can mostly only be studied by CT scanning them, and are not easily removed from the rock with acid.
At most other sites, people actively dig the layers to find fossils. They are easier to remove, and can be prepared from the rock and therefore studied much more easily. In many cases, rock can be bulk processed: dissolving the rock and seiving out the contents. In this way, it's possible to find a lot more fossils, which increases the likelihood of discovering new species and groups. The sites in Oxfordshire and Gloucestershire produce lots and lots of single teeth and bones.
However there is a plus side to the less copious discoveries from Scotland. The fossils in the Kilmaluag Formation - especially of the crucial, small-bodied animals - are often much more complete. Whereas bulk processing means most fossils are reduced to shrapnel, the method in Scotland of studying fossils with CT scanning allows us to keep the skeletons in their original position, and study them in three-dimensions, digitally. This is a huge advantage for the amount of information we can gain from each specimen. Most of the material found to date, although not from new species, represent the most complete examples of these animals known, particularly for the small reptiles, salamanders and mammals.
Looking at the collecting done by the three main groups that have worked on the Kilmaluag Formation since 1971, there are some interesting differences. Collecting in the 1970s resulted in more mammal material, whereas collecting in the early 2000s produced a lot more fish. Our work in the last decade has produced the most lepidosaur material. Overall, mammals and their close relatives, and lepidosaurs, are the most commonly found vertebrate fossils in the Kilmaluag Formation.
The numbers of different groups found in the Kilmaluag Formation is likely to shift as we identify more of the fossils we've collected, and publish those that are currently in the pipeline.
Conclusions
It's clear that the Kilmaluag Formation contains the richest vertebrate fossil material in Scotland, and is one of the most diverse and important in the world. Although at first glance it may seem less diverse than other Middle to Late Jurassic sites around the world, this is likely more to do with the way in which fossils are found, collected and studied, which limits the volume of material we can process. However, what we do have is extremely complete and provides a wealth of new information on the animals of this time period.
Comparing sites around the world with that in Scotland, we can see the distribution of animal groups in this time period, helping us understand ecosystems over 150 million years ago. There are many more fossils from the Kilmaluag Formation being studied and to be published in the coming years. This shows that protection of the shorelines of the Isle of Skye is vital to safeguard our nation's geological heritage, preserving it not only for science, but for the public, and the generations to come.
