Orcadian Lakes

The Orcadian Lakes are a series of lakes which existed during the Devonian period in the region which is now northern Scotland, Orkney and Shetland. The sedimentary rocks they left behind have been studied since the 1830's. They contain a huge variety of very well preserved fish fossils, which give a notable insight into the evolution of fish during this period.

Landscape

The landscape consisted of rounded hills formed of older metamorphic and igneous rock. The lakes varied in depth and extent from time to time, sometimes lapping against the side of the hills and sometimes retreating so that river flood plains were able to form.^[1]. Stromatolites can be found at Stromness, indicating that the lake was sometimes saline. Locally Scolecodonts (marine microfossils) have been found in the Eday Marl suggesting at least a temporary connection to the sea^[2]. The hills were bare of vegetation. ^[3]The land was not colonised by large plants as it is today. Erosion would therefore be rather rapid and probably seasonal, as would be lake productivity. This is reflected in the deep lake carbonates, which show very fine laminations, interpreted as varves.^[4]

Fauna

Fish living of the edge of the lake would float out into the centre, then sink and be preserved in the anoxic conditions prevailing at depth. The Achanarras quarry near Thurso has yielded the most extensive fish fauna and at least one example of all the following groups have been found there.^[5]

• placoderms
• acanthodians
• actinopterygians
• porolepiforms
• osteolepiforms
• dipnoans

Flora

Very few plant remains have been found and none in situ. There is evidence of algal and bacterial activity^[6]

References

1. ^ Stephen Jay Gould. The Book of Life. Ebury Hutchison 1993
2. ^ [1] Marshall, J.E.A., Rogers,D.A. & Whitely,M.J. 1996 Journal of the Geological Society, London, Vol. 153, pp. 451-466
3. ^ Stephen Jay Gould. The Book of Life. Ebury Hutchison 1993
4. ^ [2] Trewin,N.H. & Davidson,R.G. 1999. Lake-level changes, sedimentation and faunas in a Middle Devonian basin-margin fish bed. Journal of the Geological Society. 156: 535-548
5. ^ Orkney and Shetland SNH Online Publication - Life on Earth
6. ^ R. O. Muir and J. M. Ridgway, Mineralium Deposita, Volume 10 Number 3, pp 205-215

See also

• List of fossil sites (with link directory)
• Geology of Scotland
• Geology of Orkney

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