Abstract

The prolific fish faunas of the Middle Old Red Sandstone in the Orcadian Basin in northeastern Scotland are preserved in lacustrine laminites, deposited during deep-lake phases of a thermally stratified, meromictic lake. Water depth probably exceeded 75 m during transgressive lake periods which were climatically controlled with periodicites of about 20 and 90 Kyr. Fluctuating salinity levels in the lake, probably due to evaporative concentration are shown by oxygen isotopic data; these exerted environmental pressure on the fish faunas. The elevated salinity probably helped the (originally marine) fish to colonise the lake. Mass mortalities of fish were caused by abrupt increases in salinity, and de-oxygenation events associated with overturn and mixing of lake waters. The basis of food chains in the lake was cyanobacteria and planktonic algae together with plant-arthropod communities of the lake margins. Small acanthodians, Dipterus and Osteolepis were probably omnivorous and constituted prey of the larger predators Coccosteus and Glyptolepis. Colonisation of the continental lake was probably via rivers from marine areas to the south and was encouraged by developments of Devonian plant-arthropod communities. The major lake transgression at the Achanarras level provided the best opportunity for colonisation and resulted in the varied fauna of about 14 fish genera.