The Kilmaluag Formation is a Middle

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in Scotland. It was formerly known as the Ostracod Limestone for the abundance of fossil freshwater

ostracods Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typi ...

within it. The Kilmaluag Formation is very fossiliferous, with ostracods,

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. ...

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

, trace fossil burrows, and vertebrate fossil remains. Vertebrate fossils include fish, crocodylomorphs, mammals, small reptiles, amphibians and some large reptile remains including dinosaurs and pterosaurs.Panciroli E, RBJ Benson, S Walsh, RJ Butler, TA Castro, MEH Jones, SE. Evans. 2020
Diverse vertebrate assemblage of the Kilmaluag Formation (Bathonian, Middle Jurassic) of Skye, Scotland
Earth and Environmental Transactions of the Royal Society of Edinburgh nline1-22

Geology

The Kilmaluag Formation is Bathonian, and dates to around 167 million years old. It is part of the

Great Estuarine Group The Great Estuarine Group is a sequence of rocks which outcrop around the coast of the West Highlands of Scotland. Laid down in the Hebrides Basin during the middle Jurassic, they are the rough time equivalent of the Inferior and Great Oolite G ...

of the

Hebrides Basin The Hebrides (; gd, Innse Gall, ; non, Suðreyjar, "southern isles") are an archipelago off the west coast of the Scottish mainland. The islands fall into two main groups, based on their proximity to the mainland: the Inner and Outer Hebride ...

, a series of sediments laid down as the land rose and fell in the area running between what is now mainland Scotland the Outer Hebrides, causing the environment to alternate between a warm shallow sea and more exposed dry land and lagoons.Andrews, J. E. 1985 The sedimentary facies of a late Bathonian regressive episode: the Kilmaluag and Skudiburgh Formations of the Great Estuarine Group, Inner Hebrides, Scotland. Journal of the Geological Society of London, 142, 1119-37. The Kilmaluag Formation is composed of dolomitised

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

s, fine grain

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s, and mudstones, indicating that it alternated between a shallow environment, and

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons' ...

mudflat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...

s as the basin subsided and rose.Barron, A. J. M., Lott, G. K. and Riding, J. B. 2012 Stratigraphical framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf. British Geological Survey Research Report, RR/11/06. British Geological Survey, Keyworth These mudflats sometimes dried out to form desiccation cracks. The Kilmaluag Formation is unusual among the Estuarine Group for the freshwater environment it preserves, whereas many other formations in this group are predominantly brackish to marine in nature. In many beds, freshwater gastropods and bivalves can be found, including ''Viviparus'' and ''Unio'', and freshwater ostracods such as ''Darwinula''.

Fossils

Many vertebrate fossils are found in the Kilmaluag Formation, and it has been explored by palaeontologists since the 1970s, when the first mammal fossil was found there by Michael Waldman. He returned with fellow palaeontologist Robert Savage and they collected more fossils and named two new species from the area: the Docodont ''

Borealestes ''Borealestes'' was a genus of docodontan from the Middle Jurassic of Britain, first discovered on the Isle of Skye near the village of Elgol. It was the earliest mammaliaform from the Mesozoic found and named in Scotland. A second species and ...

serendipitus'', and the tritylodontid, '' Stereognathus hebridicus''Waldman, M and Savage, R.J.G 1972 The first Jurassic mammal from Scotland. Journal of the Geological Society of London 128:119-125 (although ''S. hebridicus'' is now thought to be a junior synonym to ''S. ooliticus''). Many other fossils are found in the Kilmaluag, including members of other Mesozoic mammal groups, turtles, reptiles, and amphibians.Evans, S., Barrett, P., Hilton, J., Butler R.J., Jones, M.E.H., Liang, M-.M., Parrish, J.C., Rayfield, E.J., Sigogneau-Russell, D., and Underwood, C.J. 2005. The Middle Jurassic vertebrate assemblage of Skye, Scotland. 36-39. In P. Barrett and S. Evans (eds). Proceedings of the Ninth Symposium on Mesozoic Terrestrial Ecosystems and Biota. Natural History Museum, London. The most recent discoveries in the Kilmaluag Formation include '' Palaeoxonodon ooliticus'' and '' Wareolestes rex''. and the tooth of a sauropod dinosaur. Comparisons between the Kilmaluag Formation and other sites in the UK and rest of the world suggest that the fauna represented there is globally significant, due to the rarity of fossils from the Middle Jurassic. The fauna is a subset of the animals represented in the Forest Marble Formation in England, but fossils in the Kilmaliag Formation are substantially more complete. However, most material can only be studied using micro-CT scanning making it difficult to collect and study. Exposures of the Kilmaluag Formation are protected by law as SSSIs ( Site of Special Scientific Interest) and under the new Scottish NCO ( Nature Conservation Order), and no public collection is permitted. Most fossils found to date are held in the collections of the National Museum of Scotland.

Vertebrate paleobiota

Amphibians

Turtles

Lepidosauromorphs

Choristoderes

Dinosaurs

Mammaliamorphs

References

{{reflist Geologic formations of Scotland Jurassic Scotland Bathonian Stage Marl formations Lagoonal deposits Paleontology in Scotland