Lacustrine cycles, fish ecology and stratigraphic zonation in the Middle Devonian of Caithness

Abstract

Lacustrine Middle Devonian (Old Red Sandstone) sediments show climatically-controlled cycles resulting from the long-term rise and fall of lake levels in an enclosed basin. Lithologies present were deposited in deep (stratified), shallow and impermanent lakes. Varves, the common recurrence of specific lithologies and the cyclic growth of cosmoid fish scales suggest a marked seasonal climate. Fish faunas were subject both to minor seasonal controls and more profound long-term effects. When lake level was low the faunas may have died out. When the lake overflowed, population replenishment and/or exchange with neighbouring basins was possible. Consequently fish faunas show erratic distributions (involving sudden influxes and extinctions) and no undoubted phylogenetic lineages. Discrete faunal assemblages form internally correlatable “stratigraphic zones”.