While they can be found in many other parts of the British Isles, Scotland is uniquely associated with Palaeozoic fossil fishes. That Scotland’s fossil fishes are so well known is largely thanks to a remarkable man from Caithness, called Hugh Miller. Where scholars had dismissed the Old Red Sandstone as lacking in fossils, Miller found many finely preserved fossil fishes. He published several books on field geology including, in 1841, his most famous work, The Old Red Sandstone. This eminently readable book described the formation in great detail and included dozens of beautiful engravings that illustrated the fossil fishes he had discovered.
What is the Old Red Sandstone?
The Old Red Sandstone is a distinctive set of sandstone rocks dominated by sediments laid down under non-marine, relatively dry climate conditions. It is predominantly Devonian in age, though, in Scotland at least, certain parts may be as old as Middle Silurian. This makes it much older than the formation known as the New Red Sandstone, which was laid down during the Permian.
Geologists can find Old Red Sandstone sediments across much of the British Isles, from Cornwall in the southwest of England to the Orkney Islands off the northeast tip of Scotland. For the most part, the Old Red Sandstone is indeed red thanks to the large quantities of iron oxide it contains, but, at some localities, it may be greenish-grey or purple. The fossils tend to be found in dark grey to black, muddy or silty layers present at certain horizons.
In Scotland, the Old Red Sandstone exists in two main places – the Midland Valley between Loch Lomond and Stonehaven, and the so-called Orcadian Basin area that includes, not just the Orkneys, but also the region around Caithness and Sutherland. For the most part, the Old Red Sandstone is not fossiliferous, but certain horizons have yielded fossil animals and plants, including the many spectacular fossil fishes, which are the subject of this article.
Besides the Midland Valley and Orcadian Basin regions, there are numerous smaller Old Red Sandstone outliers. One of them, at Rhynie, near Aberdeen, is of particular importance. While contemporaneous with the rest of the Old Red Sandstone, it is a chert that contains many small fossils that became silicified because of minerals from a hot spring. The Rhynie Chert, as it is known, contains an extraordinary community of bacteria, fungi, plants and animals. From these fossils, palaeontologists have been able to piece together a uniquely detailed picture of what life was like when animals and plants were making the transition from aquatic to terrestrial habitats.
What were Old Red Sandstone times like?
Geologists have described a land mass known as the Old Red Sandstone continent, which contained not just the British Isles but also the Baltic region and much of North America. During the Devonian period, the Old Red Sandstone continent was about 20˚ to 30˚ south of the equator, and the climate was hot and somewhat dry. However, there was some variation, with wetter periods that lasted for tens of thousands of years in between much drier periods. During the wet periods, green, mud-rich sandy shales were formed and these can be seen slotted in between the dry climate, red sandstones.
The animals and plants that lived at this time have been intensively collected and studied since the 1840s, starting with Miller and the Swiss palaeontologist, Louis Agassiz. While the fishes have always taken centre stage, over the years, geologists have also examined the invertebrates, primarily arthropods, and the various plant fossils and spores. Trace fossils have also been found and studied, revealing something about the behaviour of the animals that lived at the time.