Fossil fish from the north of Scotland

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.

Fig. 1. The fish beds are found in the Achanarras Fish Bed Member (formerly the Achanarras Limestone Member) and probably mark the Eifelian–Givetian boundary and consist of laminae (that is, very thin strata), which originated as non-glacial varves (annual layers of sediment or sedimentary rock). These were laid down in a lake (Lake Orcadia) during the Middle Devonian.

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.

Fig. 2. To the east of Thurso, the Ham-Scarfskerry Beds yield a vast amount of fish remains including Asmussia murchisoniana, Thursius macrolepidotus, Dipterus valenciennesia, Homosteus milleri and Dickosteus threiplandi.

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.

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 Bob Davidson(UK)

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