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The area of Caithness in the Northern Highlands is composed almost entirely of sequences of sedimentary rocks formed during the Middle Old Red Sandstone (Middle Devonian). Some of these rocks have been quarried since the 1800’s for paving slabs and building material. This has resulted in the excavation of several sites which have yielded a great quantity of fossil fish. In the 1820’s Sir Roderick Murchison published work on fossils found within these rocks. This began a long period of significant research into the area with Hugh Miller, Louis Agassiz (1807 – 1873) and others following in Murchison’s footsteps. Work has continued right through to the present, though recent work has been to determine evolutionary and environmental characteristics.
Eight sites have been recognised for their importance: Banniskirk Quarry, Holborn Head Quarry, John o’ Groats, Pennylands, Spittal Quarry, Westerdale Quarry, Weydale Quarry and Achanarras Quarry, with the latter being the most important site. All the sites lie within a Middle Old Red Sandstone sequence, where the fossil-bearing beds are composed of flagstones deposited in the deep, anoxic waters of Lake Orcadie. This freshwater lake covered much of the Moray Firth and Caithness during this time. Sediment was fed into the lake by rivers originating from highlands to the south. Although the Devonian marked a generally arid period in Scotland’s climate, the presence of varves within the lake sediments indicates seasonal climatic controls.
These sites have yielded many fossilised fish, often found in an excellent state of preservation. This is due to the anoxic nature of deeper parts of the lake – anoxic environments would have excluded scavengers and slowed decomposition. Mass mortalities also occurred, probably as a result of algal blooms.
The fish found come from the classes: Placoderms and Osteichthyes (sub-classes: Acanthodii and Sarcopterygii). The Achanarras ‘Fish Bed Horizon’ contains the most varied fauna with 14 genera and is a marker horizon throughout the area. Some of the sites, including Achanarras are also type localities.
The faunal lists includes: Achanarella, Cheiracanthus, Cheirolepis, Coccosteus, Diplacanthus, Dipterus, Glyptolepis, Homosteus, Mesacanthus, Osteolepis, Palaeospondylus, Pterichthyodes, Rhamphodopsis, Dickosteus, Gyroptychius, Thursius, Cephalaspis, Microbrachius, Watsonosteus, Pentlandia and Tristichopterus.
Trewin, N & Hurst, A. 1993. Excursion Guide to the Geology of East Sutherland and Caithness. Scottish Academic Press, Edinburgh.