Caithness Field Club

Caithness Fossil Group

Caithness Field Club Bulletin
2003

Achanarras - A Window on the Glorious Devonian
by Jack Saxon

Standing by the derelict wind-swept, waterlogged quarry of Achanarras in Caithness, Scotland it is difficult to realise that this was once a great fresh-water lake in a desert landscape in the tropics. The coral reefs of the Devonian oceans teemed with life, but Achanarras was an oasis in an arid desert, and the waters of the oasis teemed with fish. The climate was very changeable and may have flipped from a stable arid phase to a stable wetter phase many times over during the Middle Devonian period, giving rise to a cyclic sedimentary regime. Over 20 such cycles have been reported from Orkney alone.

At the beginning of the Devonian Period (410 to 335 million years ago) there were very few fishes but, during the Devonian, they began to diversify rapidly until they became the dominant life form. It was truly the Age of Fishes. A few of them even grew limbs; trackways in Easter Ross indicate that they had begun to conquer the land. They had become Amphibians. The largest known amphibian from the Devonian of Scotland is Elginerpeton from the Upper Devonian near Elgin. It was probably wholly aquatic and was long considered to be a fish. The Devonian ended with a large number of mass extinctions.

Achanarras represents probably the greatest extension of the Orcadian Lake since the fossil fauna found there occur at isolated exposures around the Moray Firth, ie Gardenstown in Banff, to Orkney (the Sandwick fish bed) and Shetland (the Melby fish bed). There are many other important Devonian sites in the world such as Canowindra in New South Wales, and Miguasha in the Province of Quebec which has been the subject of an entire book. Achanarras is, however, unique in the large number of genera and species found there. These include Dipterus (a lungfish), Glyptolepis and Osteolepis (lobe-finned fishes which gave rise to the tetrapods), Coccosteus, Pterichthyodes and Rhamphodopsis (armoured fishes), Palaeospondylus (a problematic fish with unknown affinities), Mesacanthus, Diplacanthus and Cheiracanthus (spiny fishes), Cheirolepis (a bony fish which is ancestral to all modern bony fishes), and the primitive Agnathans (jawless fishes) Achanarella and Cornovichthys. John A. Long, of Perth, Western Australia, considered Achanarras Quarry worthy of a double-page spread in his book "The Rise of Fishes". It is truly worthy of being listed as a World Heritage Site.

The Orcadian lake is interpreted as being deep, having a cold, anoxic hypolimnion (lower region) allowing organic matter to be preserved under anaerobic conditions. Mass mortalities seem to have occurred from time to time. The bodies of the fishes probably drifted from near lake margins towards the deeper part of the lake where they sank to the bottom and were covered with fine silt and subsequently fossilised. The result was the preservation of fine detail. Achanarras represents this deep water regime.

Extremely large numbers of immature Acanthodians (spiny fishes): Mesacanthus, Diplacanthus and Cheiracanthus can be found, appearing as faint smudges, often no bigger than a thumb nail. A hand lens is necessary to reveal the detail of their structure. There are, of course, mature specimens to be found and these do not need a hand lens. Palaeospondylus is almost unique in being largely confined to Achanarras. One single specimen has been recorded from the Sandwick Fish Bed. Dipterus is fairly common; ranging from 10 mm to as much as 30 mm. Coccosteus and Pterichthyodes are also fairly common.

The quarry started life as a farm quarry about 1870 to provide building stone, but it was soon discovered that the thinly-splitting flag made a good substitute for slates, and it was worked latterly almost exclusively for this purpose for many years, finally closing down in the early 1970's. The extensive quarry spoil, in which collectors are allowed to search for fossils, is the discarded material from the slate workings. The now-roofless building was erected late in the life of the quarry to provide the workers with some much-needed shelter from the rain and wind, but the roof was lost in one of the frequent gales and a wooden hut was erected nearby. The gales made short work of this as well. Scottish Natural Heritage have long-term plans at an early stage to make the ruin weather-tight and perhaps provide some interpretation of the ancient life of the quarry.

During its life the quarry had to be pumped dry every time quarrying took place and a siphon was set up to keep the water level down. Once quarrying ceased the quarry filled up with water and later trout were introduced but they are not of good quality, there being too little feeding. Nevertheless the grey herons may still be seen trying their luck with the descendants of their Devonian ancestors.

Sunday 14 April 2001 dawned to a drizzly start and, to make things worse, there was a cold breeze. This might have accounted for the poor turn-out. There were only five cars at the quarry car park. We were welcomed by Mr and Mrs Levack who farm the land through which the access road passes. Both were in their working clothes as the lambing was about to start

We assembled at the ruined building where I was able to show the party some of the material they might find, such as Dipterus, Palaeospondylus and the shadowy acanthodians. Collecting continued until about mid day when we all gathered to see what had been found. There were several good specimens of Dipterus including two on one slab. There were a few acanthodians and a possible Rhamphodopsis, but the prize for the heaviest fish went to C. Digby Grant with the head and thorax armour of Coccosteus.

There is a sequel to the visit. I returned the following Thursday with a visitor from Kemnay. The shattered fragments of a fish, which I thought had been collected on the Sunday were still lying in the window of the ruin. They looked interesting so I searched around to find all the fragments that I could and I assembled them jig-saw puzzle style and decided they were worth further interest I packed them to bring home. My colleague then proposed to visit the quarry at Skinnet I knew that the roadside quarry near Skinnet was barren but he knew of another hidden quarry. So I asked the farmer at Skinnet Farm, Mr. Calder, for permission to visit.

The quarry was smaller than Achanarras but was rich in specimens of Osteolepis panderi. I almost felt that this would have been a better venue for the fossil walk since the fishes were preserved shiny and black in a pale-weathering matrix. The rock was, however, not fissile and it would have been difficult to collect.

When I came home I assembled the fragments from Achanarras and glued them together. It did not look very elegant. I then cut the specimen down to size using a diamond saw. Most of the fish was covered with an obstinate layer of matrix. Some pieces came away easily but others had to be literally ground away. Then I discovered that the scales of the fish were large and circular, about the size of a penny. This meant that it could only be Glyptolepis. It is not an uncommon fish but is usually found fragmentary, eg a few scattered ornamented scales, a jaw bone with crocodile-like teeth, a skull covered with tiny pustules, a gular plate etc. I had never before found one displaying fully-articulated fins and scales. The specimen is far from complete and still more work has to be done on it, but it shows the very large caudal fin, the very large anal fin and the small posterior dorsal fin. My only wish is that whoever found it had searched for the missing front end of the fish and had not smashed up the tail end.