Hurricane Lakes
The Orcadian Lakes were a number of lakes that existed during the Devonian in what is now Scotland , Orkney, and Shetlands .
The sedimentary rocks they leave behind are part of the Old Red sandstone deposited about 410 to 370 million years ago and have been studied by geologists and paleontologists since the 1830s. The rocks contain a variety of well-preserved fossils , including remains of fish , which allow an insight into the development of the fish during this period.
landscape
The Devonian landscape around these lakes consisted of rounded hills made up of older metamorphic and igneous rocks . The size and depth of the lakes varied with time, so that the water sometimes reached as far as the hills and at other times receded so far that the rivers flowing into the lakes could form flood plains . In Stromness were stromatolites found allow conclusions on a temporary salinisation of lakes. In addition, scolecodonts (jaw elements of extinct annelid worms living in the sea ), which were found in the Eday marls , allow the conclusion that the lakes had a direct connection to the open sea, at least at times.
The hills were devoid of vegetation because the mainland was not yet overgrown with plants that were just developing at that time. The erosion was therefore strong and probably seasonally variable, as was the production of biomass in the lakes. This is reflected in the formation of the limestones , which emerged from the lime sludge deposited in the deep lake area , which have fine bands that are interpreted as warv stratification .
fauna
Some fish living near the shore drifted to the center of the lake after their death and sank to the bottom, where they were preserved under the anoxic conditions below . Most of these fish fossils were found in the Achanarras quarry near Thurso . At least one specimen from the following list of fish orders was found there.
- Panzerfish (Placodermi)
- Spiny sharks (Acanthodii)
- Ray fins (Actinopterygii)
- Porolepiformes
- Osteolepiformes
- Lungfish (Dipnoi)
flora
Very few plant fossils have been found in the Devonian rocks, and none of them are in a living position. There are signs of algae and bacteria activity.
Individual evidence
- ↑ a b Stephen Jay Gould (ed.): The book of life. vgs, Cologne 1993, ISBN 3-8025-1269-3 .
- ↑ JEA Marshall, DA Rogers, MJ Whiteley: Devonian marine incursions into the Orcadian Basin, Scotland. In: Journal of the Geological Society . Vol. 153, No. 3, 1996, pp. 451-466, doi : 10.1144 / gsjgs.153.3.0451 .
- ^ NH Trewin, RG Davidson: Lake-level changes, sedimentation and faunas in a Middle Devonian basin-margin fish bed. In: Journal of the Geological Society. Vol. 156, No. 3, 1999, pp. 535-548, doi : 10.1144 / gsjgs.156.3.0535 .
- ↑ Scottish Natural Heritage online publication: Orkney and Shetland - Life on Earth ( Memento of the original dated May 6, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ RO Muir, JM Ridgway: Sulphide mineralization of the continental Devonian sediments of Orkney (Scotland). In: Mineralium Deposita. Vol. 10, No. 3, 1975, ISSN 0026-4598 , pp. 205-215, doi : 10.1007 / BF00207137 .
