Plants were only able to grow near the lake border or along the rivers. This was because seeds had not developed yet and propagation by spores required an aquatic habitat.
In periods when the lake was deep, the bottom water could be hyper saline and often anoxic. These conditions were very favorable for dead fish in the lake or dead fish washed in from the rivers to be perfectly preserved as articulated specimens.
When the lake started to dry up, the environment for fish became very hostile and fish who did not manage to escape through the river system died. Because of the aerobic conditions most of the fish carcasses fell apart and are now found as disarticulated remains.
The sediments from the deep lake stage are now found as finely laminated (laminites) carbonate rich mudstones and siltstones. The sediments from the shallow to dry lake stage are found as less well laminated (or not laminated at all) mudstones, coarse flagstones or sandstone deposits. Fish in these sediments are disarticulated.
The stratigraphy of the sediments in the Orcadian Basin in the Middle Devonian (Middle old Red) can be shown in a stratigraphic table with a rough indication of the thickness. For all the groups of fishes living in the lake and or in the rivers their place in the stratigraphy will be shown.
Such a table in which the fish stratigraphy is indicated is called a biostratigraphic table. For each group of fish specific literature references will be given. Also the morphology and histology of the scales and dermal plates of each group will be presented on the general group pages (Acanthodians, Crossopterygians, Placoderms, Dipnoans).
Most of the fish living in the Orcadian Lake and its rivers have played a crucial role in the evolution of all recent fish and especially in the establishment of the first vertebrate life on land.
The fossil fishes and other faunal components are illustrated on this website with pictures of specimens from Orkney and other parts of Scotland. Since the fishes are so important in the history of palaeontology drawings of specimens and drawings of reconstructions from historically important literature have been added, together with more modern drawings of reconstructions. Each group of fishes will be dealt with on a general group page.
Links can be found at the bottom of the page with a selection of the most important books and papers dealing with the subject in general. For every group of fishes specific reference will be given at the end of the group page. The group webpage will have links to the different species found on Orkney.
Recent research has resulted in several paper where fossil Devonian fish from other countries, especially the Baltic states (Estonia, Latvia, Lithuania and Belarus) are compared. Several species from Scotland are also recorded from these states, indicating that the Orcadian Lake was sometimes connected by rivers.