Fossilised microbial deposits called stromatolites are the oldest lifeforms found in rocks across Scotland. Long before complex life appeared on Earth, micro-organisms such as cyanobacteria and algae began to form organic-sedimentary structures. These structures are produced as sediment is trapped and bound by the growth and metabolic activity of micro-organisms. They are sometimes known as ‘algal mats’ and occur mostly in shallow water environments. Shark Bay in Australia is one of the few localities worldwide where living, growing stromatolites can be seen today.
In the distant geological past stromatolites were common in both sea and lakes. So common, in fact, that the release of oxygen from these early forms of life actually aided the development of more complex life on Earth. However, it is also thought that the evolution of multicellular animal life led to heavy grazing of the algal mats, ultimately causing the stromatolites’ decline.
As well as the stromatolites found in 1200 million-year-old Precambrian-age Torridonian rocks near Lochinver in the Northwest Highlands, the life-forms have also been found in 600 million year old Dalradian rocks on the northeast coast of Islay. More recent stromatolites from the Devonian period, around 480 million years old, can be seen at Yesnaby in Orkney. These are particularly attractive fossils having the appearance of horse teeth, hence their name ‘horse tooth stromatolites’.
Colin MacFadyen, Geologist, Scottish Natural Heritage, says: These are some of the most amazing and unusual fossils in Scotland. Life as we know it would be quite different, if it were not for the micro-organisms that built the stromatolites.