Life on Earth

In the Orkney-Shetland region, the earliest traces of life are found in rocks of the Old Red Sandstone, deposited during the Devonian period. This was the Age of Fishes, but also the time when plant life diversified following the initial colonisation of land by plants and vertebrates.

Cyanobacteria and planktonic algae were the most primitive life forms in the lakes of the Orcadian Basin. Algal colonies, represented by bun-shaped masses of banded rock called stromatolites, occur at Stromness. Stromatolites usually indicate marine conditions; their occurrence at Stromness probably indicates that the Orcadian lakes were at times quite saline.

The development of land plants was controlled by their capacity to retain water. Because the lowest, moss-like, forms need permanently wet places, they could not be considered fully adapted to land. Vascular plants, able to regulate their water loss, evolved by late Silurian times and became the first true land survivors. By early Devonian times, erect branching plants with strong outer cuticles, such as Hostimella, were common. Through time, larger more complex, tree-like plants developed. However, no plants in life position are known in the Orcadian region, only small plant fragments that were washed into the lake from the surrounding landscape.

Invertebrates are poorly represented by fossils in the Old Red Sandstone succession. Only the small bivalve conchostracan Asmussia and trace fossil tracks, possibly of eurypterids (water scorpions), are known.

Fish were by far the most important and diverse form of life that existed in the area during this time. They are mainly of marine genera, but their presence in a freshwater lake environment can be explained by rapid adaptations following their initial migration at times of high lake levels, when sea connections were made. The species diversity ranges from small herbivores, mud-grubbing scavengers and omnivores to large carnivorous predators. It is most likely that these fish inhabited shallow, oxygenated lake and stream water. When they died, their carcasses drifted to the bottom of the lake. If the remains were covered quickly by sediments the fish would change, through time, from flesh and bone to rock as the lake sediments hardened under the weight of succeeding layers.

Periodically, fish suffered mass mortality, probably caused by increased salinity in hot, evaporating conditions. Fresh colonisation took place when renewed access from sea to lake was established during wetter periods.

Although body remains of higher vertebrates have not been recognised within the Orkney-Shetland region, farther south in the basin, zigzag arrangements of paired fossil foot-prints are preserved in desert sandstones, which indicate the presence of four-footed amphibians.