The Lower Palaeozoic – Cambrian, Ordovician and Silurian – the sea teems with life; 542-416 Ma

A profusion of new organisms evolved over a matter of a few million years some 540 million years ago. They all had an external, mineralised shell, which meant that they could be preserved as fossils. This was the start of the evolution of all important groups of plants and animals. The surface features of the Earth – the location of the continents – changed rapidly, and a variety of sedimentary environments came into existence. These biological and physical changes are preserved for posterity in sedimentary rocks, not least in the Cambro-Silurian succession of the Oslo region. Throughout the Cambro-Silurian – the collective term for the Cambrian, Ordovician and Silurian periods – Scandinavia was located on the Baltic Plate. This plate, which formed late in the Precambrian, was alone and surrounded by a deep ocean at the threshold to the Cambrian, but Baltica and the American plate Laurentia gradually drifted towards each other. The Caledonian mountain chain was formed in this collision zone as the Iapetus Ocean closed. Paleotethys, another deep ocean, nevertheless still separated the plate from the supercontinent of Gondwanaland to the south. Throughout this time, Baltica drifted from about 60 ºS to approximately 30 ºS towards the end of the Silurian.