Kilmaluag Formation

The Kilmaluag Formation is a Middle Jurassic geological formation in Scotland. It was formerly known as the Ostracod Limestone for preserving an abundance of fossil freshwater/low salinity ostracods. Gastropods, bivalves, trace fossil burrows, and vertebrate fossil remains have also been recorded from the formation. Vertebrate fossils include fish, crocodylomorphs, mammals, small reptiles, amphibians, theropod and sauropod dinosaurs and pterosaurs.

Geology

The Kilmaluag Formation was deposited during the Bathonian stage ~167 million years ago and is part of the Great Estuarine Group. Like other rock formations within the Great Estuarine Group, the Kilmaluag Formation is composed of a series of fine grained sandstones, silts, mudstones, and dolomitised limestones. The sediments were deposited in a freshwater influenced low salinity closed lagoonal palaeoenvironment subject to lagoon margin transgressions and regressions. During regressions, sediment abandonment ensued and is indicated by the deposition of dolomitised limestones, in which large desiccation cracks can be observed. In contrast, during transgressions, sediment influxes and increases in water depth led to the deposition of finer grained silts and muds. Occasional shoreline conditions are indicated by rippled sandstones.

The Kilmaluag Formation is divided into two facies: the clastic facies deposited in the Sea of the Hebrides basin of northern Skye, and argillaceous limestone facies present in the Inner Hebrides basin, present on the Straithaird peninsula, Isles of Eigg and Muck.

Fossils

In many beds, freshwater gastropods and bivalves can be found, including Viviparus and Unio, and freshwater ostracods such as Darwinula and Theriosynoecum.

Many vertebrate fossils have been recorded from the argillaceous facies of Straithaird since the 1970s, when the first mammal fossil were found by Michael Waldman. He returned with fellow palaeontologist Robert Savage and named two new species from the area: the Docodont Borealestes serendipitus, and the tritylodontid, Stereognathus hebridicus (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. Notable vertebrate fossil recent discoveries in the Kilmaluag Formation include Palaeoxonodon ooliticus and Wareolestes rex. A notable dinosaur find includes the tooth of a sauropod dinosaur. The most recent scientifically pre-published find includes a pterosaur found in 2006.

Comparisons between the Kilmaluag Formation and other British and global Middle Jurassic localities suggest that the fauna represented is globally significant, due to the scarcity of similarly aged sediments. 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.

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), thus 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

Pterosaurs

Mammaliamorphs

See also

• List of fossiliferous stratigraphic units in Scotland