Fossil fishes of the Old Red Sandstone of Scotland

Dr Neale Monks (UK)

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.

Broadly, Old Red Sandstone times were arid, a lot like the Australian outback. However, the climate was seasonal and, like many rivers and lakes in Australia, water level would rise and fall through annual cycles. Sandstorms were common and some of the sand would end up at the bottom of rivers and lakes. Sometimes entire sand dunes would be dumped on lakes, killing the animal life in the lake entirely. Bad news for the fish, but great news for fossil collectors – such events have produced some remarkably preserved fossil fish.

Elsewhere, fossils were formed in the anoxic (that is, oxygen-poor) muds at the bottom of lakes. Because there were no animals and few bacteria living at these depths, organic material was well preserved. Eventually, such sediments became shales. In some cases, the fossil fishes found here are complete skeletons or in large groups, implying they were preserved quickly, before their bodies had time to decompose. However, in other cases, the fossils are fragmentary, perhaps limited to just the odd scale or tooth. In such cases, the dead fish probably drifted about for a while, gradually decomposing, before it was finally preserved.

Not all fossil fishes come from deep water habitats and it seems likely that, at times, conditions in the shallows became anoxic as well. By analogy with modern lakes, this could well have been because of eutrophication, the process whereby blooms of algae consume all the oxygen in the water, killing the fish.

Where to collect

The oldest part of the Old Red Sandstone that yields fossil fish is the Cowie Harbour Fish Bed, which is exposed along the foreshore between Stonehaven and the small village of Cowie to the northeast. This is part of the Midland Valley portion of the Old Red Sandstone and is Silurian in age. Fossils are found in fine-grained shales and mudstones in between layers of sandstone. While the fossils of the freshwater crustacean Dictyocaris are common enough, fossil fish are very rare indeed. I can recall collecting from this formation several times, while studying at nearby Aberdeen University, and never once found any fish remains.

Fossil collecting is more productive further north and the Orcadian Basin portion of the Old Red Sandstone is exposed at several sites along the East Sutherland and Caithness coastline, as well as at inland quarry sites. The fish lived in a big freshwater lake, known as Lake Orcadie, which covered what are now Shetland, Orkney, northern Scotland and western Norway.

Probably, the best-known fish-collecting site is Achanarras Quarry (Fig. 3) , near Spittal, Caithness. The quarry is now run by Scottish Natural Heritage and is clearly marked by road signs. It even has its own car park and information centre.

The fish are found in flagstones that were once quarried for roofing slates. Great piles of spoil are all around and collectors mostly split these flagstones, hoping to find fossils inside them. To date, some 14 different species of fish have been found there, including a small (typically 1cm to 2cm) fossil fish known as Palaeospondylus gunni, first found at this site and only rarely found anywhere else. There is some debate about precisely what Palaeospondylus gunni might be, but current thinking is that it is a larval lungfish of some type.

To be fair, while Achanarras is well known for its fossil fish, there are so many visitors to the site that getting good quality fossils isn’t easy. The flagstones contain numerous thin layers and the fossils are found only in certain silt and shale horizons, collectively known as ‘fish beds’. You need to split the flags using a geological hammer and, since that is likely to send small bits of rock flying all over the place, goggles must be warn to protect your eyes. Splitting the flags is easy enough, but it takes a certain amount of practise to learn which horizons have the most fossils.

Other inland collecting sites include Spittal Quarry (very near Achanarras) and Holborn Head Quarry, near Thurso (Fig. 4). These are working quarries, so permission needs to be obtained before visiting. Ideally, visit these sites as part of a properly organised group excursion.

Among the coastal localities where fossil fishes can be found are the exposures at Thurso, John O’Groats (Fig. 5) and, less abundantly, Tarbat Ness (Fig. 6).

In particular, Thurso is almost as famous a locality as Achanarras and, while it isn’t a good place for complete specimens, individual scales and armour plates are very common. As at Achanarras, the aim is to split flagstones and so reveal fossils that are found only at certain horizons.

A bit further south, fossil fish can also be found at Eathie, a few miles south of Cromarty. This locality is perhaps best known for its exposure of Jurassic rocks and is one of the few places in Scotland where ammonites can easily be found. However, below the Jurassic beds is the middle part of the Old Red Sandstone and this contains nodules that sometimes have fish remains in them. In Miller’s day, whole skeletons were found, but nowadays collectors will mostly find isolated scales.

Devonian fish diversity

The fishes of the Old Red Sandstone come from several different groups – two families of acanthodians, four families of placoderms and four families of bony fish. Noticeably absent are cartilaginous fish and jawless fish like those found in marine deposits of equivalent age, though Palaeospondylus gunni has been placed in both groups, at one time or another.

The acanthodians were an odd group of fish that shared features in common with both cartilaginous fish and bony fish. They evolved in the late Ordovician, reached their zenith in terms of diversity during the Devonian and then slowly declined, becoming extinct by the end of the Permian. In general shape, they were strikingly shark-like, with streamlined bodies and non-symmetrical tail fins. They also had a skeleton composed primarily of cartilage. However, appearances seem to be misleading when it comes to acanthodians and their finer skeletal and anatomical details suggest they were, in fact, more closely related to the earliest bony fish.

Acanthodians are divided into three orders and numerous families. At Achanarras, five species from two families are known. Three species come from just one genus, Diplacanthus (Fig. 7). These were fairly small acanthodians, getting to around 12cm or so, and seem to have swum about in mid-water catching smaller fish. Diplacanthus fins were armed with stout bony spines and, consequently, some ichthyologists refer to acanthodians of this type as “spiny sharks”.

The other two acanthodians are Mesacanthus peachi and Cheiracanthus murchisoni. M. peachi is a very small species, reaching about 5cm in length, and probably fed on plankton. It was, in turn, a major prey item for many of the other fishes in the lakes and rivers where it lived. By contrast, C. murchisoni was quite a big fish, with the largest specimens being almost 30cm in length. It was presumably a middle-ranking predator, eating smaller fish, but, in turn, being taken by the bigger lungfish and lobe-fins.

The placoderms were a group of fish that had heavy armour around the head and forequarters. Most were predators, but they lacked teeth. Instead, the armour plates along the jaws had sharp edges and these were used to grab and slice their prey. Because of their heavy armour, most lived on or near the substrate. The most famous of the Old Red Sandstone placoderms is probably Pterichthyodes milleri, a member of very successful group of placoderms known as the antiarchs. P. milleri was a medium-sized fish that got to about 15cm or so in length and is usually found in places believed to be rivers rather than lakes. Its armour was very stout, covering not just its head and thorax, but also its pectoral fins.

Another of the Old Red Sandstone placoderms is Coccosteus cuspidatus, a member of a group of placoderms known as arthrodires that became highly effective, mid-water predators. Unlike the antiarchs, the arthrodires had a jointed ‘neck’ in their armour that allowed them to open their jaws more widely.

The bony fish group, Osteichthyes, is usually divided into two groups – the actinopterygians (or ray-finned fish) and the sarcopterygians (lobe-finned fish). In the modern world, virtually all bony fish are actinopterygians, including such things as eels, herrings, trout, catfish, carp, seahorses, cichlids, flounders and pufferfish. Though important from a scientific perspective, in terms of ecological importance, the sarcopterygians barely register at all, amounting to a handful of lungfish species in South America, Africa and Australia, and two coelacanth species living in the Indian Ocean.

Both actinopterygians and sarcopterygians are represented in the Old Red Sandstone fish fauna. The only actinopterygian in the Old Red Sandstone is Cheirolepis trailli, a species that got to about 30cm in length. Their jaws were quite large and filled with numerous small sharp teeth. By analogy with modern fish, these streamlined fish were probably open water predators.

The sarcopterygians are rather more diverse. One of the best known species is Osteolepis panderi (Fig. 10), a robust, elongate fish that got to about 15cm in length.

This species is very abundant at Holburn Head Quarry and palaeontologists have noted that their distribution suggests mass mortality events, perhaps because of sudden changes in salinity or oxygen availability. At other places, including Achanarras, a larger species called Osteolepis macrolepidotus (Fig. 11) can be found. Both species are members of a group of lobe-finned fish known as the Osteolepiformes. Although Osteolepis were not the ancestors of those fish that evolved into amphibians, they were closely related to them, and more so than either lungfish or coelacanths.

Coelacanths are not found in the Old Red Sandstone, but lungfish are. The species Dipterus valenciennesi is quite common, usually in the form of isolated scales, but also as articulated specimens in the Sandwick Fish Bed at Cruaday Quarry on Orkney. At well over 30cm in length, Dipterus valenciennesi was quite a big fish, but it had peculiar crushing plates in its mouth that suggest it mostly fed on shelled invertebrates rather than fish. In this sense, Dipterus valenciennesi is remarkably similar to modern lungfish.

Hugh Miller subtitled his book on the Old Red Sandstone, New Walks in an Old Field, and it is certainly true that anyone collecting fossils from the Old Red Sandstone will be able to see glimpses of a very ancient world. The Devonian period was a time of change, with vascular plants, arthropods and vertebrates each making the transition from the aquatic environment to the terrestrial. While Osteolepis and Dipterus were not our distant pioneering ancestors, they were closely related to them and shared many features in common with them. Perhaps it is that which makes collecting from the Old Red Sandstone so rewarding.

Hugh Miller

Hugh Miller was born in Cromarty in 1842. He was entirely self-taught and never acquired any academic credentials. He worked as a stonemason for some 15 years, until dust from the quarries caused him such pain that he left the trade and worked as a clerk in a bank. Possessing a brilliant mind, he wrote important books about folklore as well as fossils and, from 1840, was editor of the newspaper of the Free Church, The Witness. A committed evangelical Christian throughout his life, Miller never doubted the great age of the Earth, but neither did he doubt the guiding hand of the Creator. He was consequently sceptical of the ideas about evolution emerging at the time. Suffering from hallucinations and terrible pains in his head, and fearing that he might harm his family, Miller shot himself in 1856. The Hugh Miller Museum Birthplace Cottage in Cromarty is well worth a visit. It is run by the National Trust for Scotland and contains numerous fossils, books and other artefacts.

Further reading

Scottish Fossils, by Nigel H Trewin, Brocken Dunedin Academic Press Ltd, Edinburgh (2013), 118 pages (hardback), ISBN: 978-1-780460-019-2