5 The Devonian Period
Before going any further, click on 'View document' below and read pages 76-77 from Douglas Palmer's Atlas of the Prehistoric World. Do not worry too much about all the different names of fish groups in this, the 'Age of Fishes'.
Environmental change is known to have a significant impact on the evolution of life. For example, widening oceans generate barriers between populations and promote increasing genetic divergence between them over time. Narrowing oceans, rising mountain belts and the formation of large continental areas can also have major effects, causing both evolution and extinction. During the Silurian, the Iapetus Ocean had been narrowing (Atlas, p. 72), allowing the progressive mixing of marine organisms that had previously been separated by a wide oceanic barrier. By the Devonian, the continental areas of Laurentia, Avalonia and Baltica had amalgamated to form a large land mass, with huge rivers and lakes that were colonised firstly by jawless fish and then by predatory jawed fish. (Jawed fish had first evolved in the Ordovician (Atlas, p. 71), but only became abundant in the late Silurian and Devonian, evolving into many different marine and freshwater groups.) The rising mountain chains separated river systems and promoted the evolution of different species of fish in different inland waterways. Likewise, land plants were spreading globally, evolving distinct floras in particular regions with different climates.
We now look at one of the most significant developments in the evolution of our vertebrate ancestors - the first tetrapods in the late Devonian.