Vertebrates

Acanthodians

Actinopterygians

Crossopterygians

Dipnoans

Placoderms (antiarchi)

Placoderms (arthrodira)

Palaeospondylus gunni

Invertebrates and plants
Stromatolites

Arthropods

  Plants

Ancient lake sediments forming the cliffs of west Mainland

  Bibliography

  Acknowledgements


"I traced the formation and upwards this evening along the edges of the upturned strata, from where the great conglomerate leans against the granite, till where it merges into the ichythyolitic flagstones; and then pursued these from older and lower to newer and higher layers, desirous of ascertaining at what distance over the base of the system its more ancient organisms first appear, and what their character and kind And, embedded in a greenish colour layer of hard flag, somewhat less than 100 yards over the granite, and about 160 feet over the upper stratum of the conglomerate, I found what I sought, -- a well marked bone, -- in all probability the oldest vertebrate remain yet discovered in Orkney."

The Asterolepis of Stromness  - Hugh Miller

The information and images on these pages were assembled by Jan den Blaauwen. Certain pictures and drawings on this site are copyright protected. For these pictures and drawings, the normal rules for copyright apply.

 

  Palaeozoic life

Life in a Middle Devonian Lake



Lake Orcadie is one of the best examples in the world of a Middle Devonian lacustrine environment with a high potential for preservation of its different faunal components. During the Middle Devonian (Middle Old Red Sandstone period) this area with one or probably more interconnected lakes was situated south of the equator. The fossil fish of Orkney will be highlighted here.

The rocks found in the Orkney Islands are part of the Orcadian Lake basin which extended at this time from Greenland through Shetland as far as the Moray Firth area (see literature, list of papers, books).

Many different species of fish were living in the lake or in the rivers which were flowing into the lake from the north and the west. It is also possible the lakes also drained south-eastwards to the sea.

Due to shifts in the climate, the lake could change from a relatively deep extended lake to several smaller interconnected lakes and eventually to a completely dried out lake basin. The sediments on Orkney show these (cyclic) climate changes clearly. Well laminated sediments indicate a deep lake facies while sediments showing mud cracks and current ripples are found in shallower marginal to dry lake conditions.

                   Lake sediments                                                Mud cracks                                                Ripple marks

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 favourable 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 stratigraphic table 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.

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. Links can be found to a reference page with a selection of the most important books and papers dealing with the subject.