The north of Scotland is famous to scientists and amateur collectors for its wealth of localities where fossil fish of Devonian age can be collected. From plate tectonics, we know that in Devonian times Scotland was situated just below the equator, as part of a continent that was largely arid desert and where land plants were only just emerging. Most life on earth was still aquatic and fishes were the most successful backboned animals.
The fossil fish of the area are unique in many ways. They present a window on the development of vertebrates, in which many of the innovations necessary to pave the way for the next great evolutionary step (the invasion by tetrapods of the land) were already in place. The fauna contains the acanthodians, one of the first group of vertebrates to evolve jaws, and the lobe finned fishes, so called because of their fleshy lobes supporting their pectoral and pelvic fins. The lobe fins also include the lungfish. Their fleshy fin lobes played an important role in the development of the limbs of early four-legged animals (tetrapods) and ultimately to all terrestrial vertebrates today – including ourselves.
The classic Middle Devonian (380 to 375myrs old) locality is Achanarras Quarry in Caithness, where exquisitely preserved fish can be collected in an old roof tile quarry. Many such quarries existed in the past and fish have been widely collected from several localities over the years. The fish are preserved in thinly laminated siltstones and limestones, and this has probably become to be accepted as the normal mode of preservation for the area, whereby the fish died and eventually sank to the bottom of a deep lake in what is known as a lacustrine setting. Fig. 1 illustrates the position of the ancient lake, dubbed the Orcadian Lake (or Lake Orcadie) and shows the deepest part of the lake extended from the tip of the mainland and covered the present day Orkney and Shetland Isles.
The great depth of the lake contributed to the excellent preservation of the fossils, probably due to low oxygen levels on the lakebed and therefore less scavengers and bacterial activity. The fish carcasses lay largely undisturbed in a low energy environment (that is, with an absence of currents) and were gradually covered by river and wind borne influxes of silt and organic matter. These fell through the water column and, over time, were buried at depth in a great thickness of laminated sediment, which is today found as limestones and siltstones. However, the lake was not static and, periodically, the level would rise and fall, depending on the processes that fed water into, and drained it from, the lake.
To the south, the rivers that fed the lake deposited sediments in lowland areas, known as alluvial plains and, during periods of high lake levels, these arid landscapes would be flooded giving rise to semi-permanent, comparatively shallow verges to the lake. This nearshore area we know today as the Moray Firth and here exist equally celebrated fish beds of the same Middle Devonian age, but with a quite different process and mode of fossil preservation.
‘Nodule’ bed localities
The nearshore environment was rich in calcium carbonate, probably derived from the mineral rich sediments of the underlying alluvial plain, and gave rise to a form of preservation commonly known as nodules. These are round to oval, sometimes flat or irregular, smooth shaped carbonate clasts, which can vary in size from around 5mm to 600mm. Note that the term ‘nodule’ is technically incorrect, as this type of preservation is accurately called a concretion. However, over time, these two terms have become interchangeable when discussing the Scottish localities and therefore ‘nodule’ will be retained for the purposes of this article.
The Achanarras fauna
It is estimated that 18 or 19 species of fossil fish and a single arthropod make up the Achanarras fauna throughout the Achanarras horizon. Recent discoveries have augmented this number from a previous figure of around 14 to 16. The fish forms represented are;
- Acanthodians or ‘spiny sharks’.
- Agnathans or jawless fish.
- Ray fins.
Fish preserved in nodules
Whereas today’s collector would usually expect to gather specimens in the classic fine grey flagstone matrix typical of the northern quarries, the Moray Firth localities still yield nodules and some of them contain the fossilised remains of fishes. However, most nodules do not contain remains and considerable investment of time is required to collect a single fish-bearing nodule. The reason is that, like Achanarras Quarry, the fish are preserved only at certain levels within the nodule bearing outcrop, where the nodules occur in both limestones and relatively soft clays. Collecting from the outcrop is prohibited by the Scottish Fossil Code. However, collecting from talus, shingle and stream beds is allowed and, in these instances, both barren and fish bearing nodules derived from the full thickness of the outcrops are mixed together, with few differentiating features. Therefore, every nodule found should be investigated onsite to determine whether it has potential.
In the Orcadian Lake, preservation in concretions (nodules) occurred in shallow water near the lake margin, when the lake was at its deepest and transgressed the alluvial plain environment.
Fish preserved in nodules can be more disarticulated and incomplete compared to those preserved in deep lake siltstones and limestones, and individual spines, scales or dermal plates may be all that is preserved within the nodule. This is perhaps due to a greater abundance of scavengers and nearshore currents. However, such specimens can be equally valuable scientifically, as some features can be more clearly exposed.
The chemistry involved in the formation of concretions enclosing fossils is not fully understood and appears to vary considerably, and laboratory experiments have so far failed to reproduce concretion formation (McCoy, 2013). Preservation also varies from the spectacular 3D preservation of the occupant, for example, in the Santana Formation in Brazil, to the situation seen in Scottish deposits, where the fish is completely compressed.
An abundance of calcium carbonate is required for the calcareous nodules to form and this, to an extent, may be derived from mineral rich sediments (caliche) within the transgressed alluvial plains. It is perhaps augmented by carbonate deposition by lake margin plants (Gierlowski-Kordesch, 2010) and concentrated by evaporation. Fish would die and sink to the lakebed where, presumably, burial would be more rapid due to the proximity of sediment supply from the rivers that fed the lake. While still in the soft sediment phase and shortly after burial, it is presumed that carbonate ions, in solution, migrate towards the carcasses, cementing the grains of sediment and forming a jacket around the fish. Once buried at great depth, the sediment is compacted and lithified, and the nodules generally become harder than the surrounding sediment. It has often been reported that nodules containing fish are fish shaped. In Lake Orcadie deposits, the nodules tend to be round to oval, except in specific deposits, for example, Tynet Burn Upper Nodule Bed, in the bottom and middle units.
The size range of, and preservation within, these nodules can vary greatly. At Tynet Burn, pea-sized nodules can contain a single fish scale, while at Edderton, nodules weighing many kilos can enclose several fish. The figure demonstrates the unique preservation mode and nodule formation and disruption. This is typical of Tynet Burn nodules, where deformation of the soft sediment during water release events, followed by localised tectonic events, resulted in the nodules being broken and re-cemented by carbonate infilling.
As previously stated, the bulk of all nodules are barren and this means that the reason for their formation seems more difficult to explain, there effectively being no nucleus present. However, one reason may be that formation commenced around a mineralised event or that the decomposition process continued after the nodule formed, eventually leaving no trace of the original carcass.
My colleagues from Aberdeen University and I have excavated all the known nodule localities with the permission of both the landowners and Scottish Natural Heritage, and this work has shed light on questions surrounding the apparent different types of nodules, as illustrated in Fig. 3. It can be seen that the preservation at Edderton, Eathie/Cromarty and Gamrie comprises dark bone on a grey matrix, similar to that observed in the deep water siltstones. On the other hand, the nodules from Tynet Burn and Lethen Bar appear quite different. They are unusual in that the fish are preserved in crimson and purple colours and, over the years, this has rendered these specimens very desirable to collectors. This type of colouration is typical of iron compounds, but why would these localities yield fossil bone preserved in this way?
The answer was realised during examination of Tynet nodules under the microscope, when thin sections revealed microscopic remains of fossilised filamentous bacteria in calcite. It became clear that, after burial, these anaerobic forms of bacteria were processing the fish carcasses by metabolising the naturally occurring iron in the organs and tissues with the by-product iron oxide being deposited, thereby giving the fossils their unique and exquisite colours. Furthermore, at sites within the carcass where iron was concentrated in blood rich organs (the kidneys, liver and heart), a higher concentration of deposited iron oxide reveals the position of these organs in rare cases.
Because of their unique attraction, all the sites have been stripped over the last 150 years and yield little or no loose material today. Fossils can still be found in nodules in shingle at Cromarty and Eathie, and responsible collecting is encouraged by information boards there. However, all of the localities are sites of special scientific interest (SSSI) and special access permission is also required at Tynet Burn, Gamrie and Edderton. The Lethen Bar site was worked commercially for lime in the nineteenth century and its exact location is not currently known. The Tynet Burn site is now completely overgrown and requires excavation to yield more material or to examine the outcrop. However, excavation of the outcrops is not allowed at any locality without robust scientific justification.