Invertebrates and plants
Ancient lake sediments forming the cliffs of west Mainland
"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
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.
Life in a Middle
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.
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
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
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
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.