Discovery of the arthrodire genus Actinolepis (class Placodermi) in the Middle Devonian of Scotland

A single specimen of the placoderm Actinolepis found in the Middle Devonian (Eifelian) Achanarras fish bed of Achanarras Quarry, Caithness, is the first record of the genus in the Devonian of Scotland. The articulated specimen displays an external view of the dorsal armour. Two species of Actinolepis are known from the Middle Devonian of the Baltic: Actinolepis tuberculata and Actinolepis magna. This find strengthens the faunal connection between the Baltic area and northern Scotland in Eifelian and Givetian times, which is supported by other species common to both areas, such as the arthrodire Millerosteus minor. Both areas were connected, perhaps at different times, to the same marine area. The most likely connection is to the Rheic Ocean to the south, but another possible connection lies between Laurentia and Baltica to an ocean to the north. Migration of fish into the Old Red Sandstone continent was controlled by long-term climatic cycles. When lake basins overflowed at times of high runoff, fish could migrate to internal continental basins through river systems. During dryer and hotter periods lake basins became isolated and fish evolved in non-marine waters, isolated from marine conditions, possibly explaining the minor differences in fish species from widely separated basins.