Fossils are the remains of once living organisms. They can be anything from the original shells and bones to their burrows and footprints. Scotland has a long and fascinating geological story. It has taken almost 3000 million years and a journey around the globe, welding together parts of several of the Earth’s tectonic plates, to become what we know today as Scotland. Throughout this time, it has witnessed many of the significant changes in life on Earth, from the smallest microbes to the mighty dinosaurs, many of which are now extinct.
In the past, Scotland has been a desert, a tropical swamp, a mountainous landscape with volcanoes, and an ocean floor, all of which have been witness to several ice ages. We know this from the rocks, and the fossils they contain, that are all around us and beneath our feet.
Geologists (who study rocks) and palaeontologists (who study fossils) have divided time into manageable chunks separated by extinctions, climatic change and other world changing events.
Scotland was part of an ocean floor that existed far south of the equator during the Cambrian. The shallower waters of this ocean produced sediments that are still preserved in northern Scotland. A sequence of sandstones, limestones and mudstones around Assynt, south of Durness and on the Isle of Skye contain the fossils of strange shelled and segmented animals called trilobites.
Life teemed in the shallow ocean (named the Iapetus Ocean) of the Ordovician period. Scotland lay far to the south of the equator not far from the southern edge of a huge continent called Laurentia. The fine-grained muds, now shales, of the Southern Uplands formed in the deepest water of the ocean’s basin.
In the deeper parts of the ocean, serrated stick-like animals called graptolites thrived. Graptolites are extinct today, but their fossils are commonly found in the shales of the Southern Uplands.
In rocks near Girvan, where the sediments were being deposited on the ocean’s outer shelf, biodiversity was greatest. Many different forms of trilobites, starfish, bivalves and brachiopods, as well as some other stranger animals were all part of a fauna that came together as a result of undersea landslips. Scotland’s largest trilobite comes from here.
The Iapetus Ocean disappeared by the end of the Silurian period. The great continent of Laurentia collided with another continent called Baltica. What is now England was on the northern coast of Baltica. The closure of this ocean brought England and Scotland together for the first time during Silurian times.
Before the ocean had finally closed, northern Scotland lay above sea level and southern Scotland constituted the ocean floor. Sand and mud was washed into the ocean from the land and formed very thick layers of shale and muddy sandstones (known as greywackes). The youngest Silurian rocks show the change from sedimentary rocks formed in a marine environment, through to rocks formed on land, in rivers and lakes – as the land emerged from the sea. Silurian rocks form a large part of the Southern Uplands. The fossil remains found include trilobites, shellfish, huge sea-scorpions (known as eurypterids), pod-shrimps and early jawless fish. The earliest animal to have walked on land was a millipede that was found in rocks near Cowie, near Aberdeen.
Scotland was still south of the equator during the Devonian period, but was steadily moving northwards. All of Scotland was above sea level at this time and the environment was almost like a desert with some northern mountains as high as the Swiss Alps today, and perhaps even as high as the Himalayas. Nearly 400 million years of erosion has reduced these mountains to the hills we know today.
Ephemeral lakes trapped fish that are preserved in ancient dried-up pools where they died at Dura Den in Fife.
Fish flourished in the rivers and lakes from Shetland, Orkney and Caithness, Moray, and south into the Midland Valley of Scotland. It is from rocks of this age that the first lungfish are known although it is another few million years after the Devonian before the first land living amphibians are found.
Scotland was volcanically active at this time with hot springs around Aberdeen that have preserved, in exquisite detail, some of the earliest land plants and animals at Rhynie.
During the Carboniferous, Scotland moved from the equator to just north of it. It was a time that saw lush forests in an equatorial to tropical climate. The northern parts of Scotland were upland areas and the southern parts of Scotland were lower lying.
During the Carboniferous, the sea level rose and fell several times. Scotland was frequently inundated by warm shallow seas, producing sediments of thick fossiliferous limestones. At other times it was blanketed by tall trees and plants, which produced much of the coal we mine today.
The shallow tropical seas and lakes of Carboniferous Scotland, were full of life. Some of the world’s most spectacular fossils of this age are found here. The Bearsden Shark is the best preserved shark of its kind ever found and is associated with shrimps that still preserve their muscles and blood vessels! The land was covered by tropical swamps where forests of large fern trees and club-mosses flourished. Giant centipedes, dragonflies and spiders ruled the landscape along with amphibians and early reptiles, the forerunners of the dinosaurs.
By the early Permian, all the continents of the world were joined together in one vast super-continent called Pangaea. Scotland lay ‘sandwiched’ between America and Europe and moved further north into the arid climates of the sub-tropics. Fossils are very rare from this period, but the footprints of animals crossing the sand dunes have been found in places such as Hopeman, Elgin, and Dumfries. The hollowed-out skeletons of early mammal-like reptiles have been found near Elgin that are very similar to animals found in rocks of the same age in South Africa.
Desert conditions continued into the Triassic period in Scotland although the climate was becoming wetter towards the end of the Triassic. Sea level also began to rise inundating some of the flat lying areas of Scotland. Fossilised salt crystals in mudstones suggest that sea-water covered parts of Arran at different times although it evaporated too fast for shelled animals to colonise. Footprints from the Isle of Arran of a precursor to the dinosaurs, called Chirotherium, have recently been found.
Shallow warm seas surrounding the fringes of Scotland were full of life. Shellfish, corals, ammonites, belemnites and marine reptiles such as ichthyosaurs and plesiosaurs, were common in the shallow seas and lagoons. Many of these Jurassic sea creatures are now extinct. More mountainous regions of the Highlands remained well above sea level. Scotland at this time was about 40° north of the equator (where Greece is today).
Dinosaurs and small primitive mammals roamed the landmasses of Scotland. A few fossilised dinosaur bones and footprints have been found in Scotland.
A Jurassic dinosaur footprint from Skye photographed in natural light (left) and a false colour image of the same footprint, created with a laser scanner, showing relief (right)
During the Cretaceous Scotland was mostly covered by a shallow, warm sea producing a blanket of white chalk (made from the skeletons of tiny marine animals).
Most of the Cretaceous rock has since been eroded during the Palaeogene, Neogene and Quaternary periods. Today, only very small exposures of Cretaceous rocks remain.
Before the Palaeogene, there was no Atlantic Ocean as Europe and America were part of the same vast continent. Because of to the movement of the Earth’s tectonic plates at the beginning of the Tertiary, America began to move westwards and Europe eastwards. As the continents continued to separate, the split between them continued to grow wider and eventually became the Atlantic Ocean. Scotland was very active volcanically at this time, especially in the west. As a result, there are not many fossils of this period known except where sediments have deposited between lava flows allowing leaves of trees to accumulate.
The story of Scotland during the last 2.6 million years is one of ice sheets, glaciers and warmer periods. It is dominated by the erosive power of the ice rather than the deposition of sediments. This activity has shaped mush of the landscape of Scotland that we see today.
Ice ages have come and gone repeatedly with the last major ice age in Scotland peaking around 18000 years ago. Ice up to 1km thick flowed across the country, scraping rock, gouging U-shaped valleys and eroding mountains.
Rarely are there pockets where sediments are deposited that contain the fossil (or near-fossil) remains of the animals and plants that lived in these hostile environmental conditions. Woolly mammoths, rhinos and other large mammals roamed the Scottish tundra in areas now covered in sea to the northwest of Scotland. These animals have also been found in Ayrshire. Some incursions by the sea deposited shell banks that include snails, bivalves crabs and the bones of other marine animals in Dunbartonshire and elsewhere.
Scotland is currently still heading north and it is predicted that it will continue to do so over the next 200 million years or so. Over this period, much can happen, but it may be that the mountains will erode further and the future potential for the formation of fossils will diminish, especially if Scotland is further eroded by glaciers from further ice ages.
Neil Clark is curator of palaeontology at the Hunterian. All the images in this article are from the Hunterian collection.