Australia: The Land Where Time Began

A biography of the Australian continent 

Achanarras Fish Beds, Caithness, Scotland.

The Old Red Sandstone fossil fish beds from the Devonian of Scotland have been known of since the 1830s when the book Recherches sur les Poissons Fossiles was published by Agassiz. The deposits covered the area that had been covered by the Orcadian Lake on the southern margin of the Old Red Sandstone Continent that in the Middle Devonian covered the Moray Firth, Orkney and Shetland and much of Caithness. The fish lived in the shallow water around the margins of the lake where the water was warm and well oxygenated. When fish died at least some drifted to the centre of the lake before sinking to the lake bed where the water was deeper, colder, and beneath a thermocline, was anoxic, being preserved in the laminated muds on the bed of the lake, a situation that was similar to that of the Crato Formation.

The authors¹ suggest that algal blooms may have caused the water to become anoxic, leading to high concentrations of fish carcasses on the lake floor during mass mortality events (fish kills), the result being that as there would have been no scavengers in or on the bottom sediments because of the anoxia so the bodies of the fish could have been preserved in good condition. Included in the fauna were placoderms, agnathans, acanthodians, and bony fish, that included ray-finned fish and lungfish (Trewin, 1985, 1986).

Sources & Further reading

1. Selden, Paul & Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing.