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Abstract

The term ‘Devonian’ was first used in the 1830s to describe a succession of rocks in Devonshire, a large county in SW England. The Devonian period is an interval of about 57 Ma in the middle part of the Palaeozoic with remarkable geological variety. It was influenced by active plate movements producing a wide range of environments and facies. The geosphere and biosphere were undergoing extraordinary changes during this period which can claim a remarkable number of biotic ‘firsts’, including the first appearance of vascular plants, ammonoids, insects and amphibians. A major radiation of fish took place and the Devonian has therefore been called ‘the age of fish’. The development of the earliest land floras, forests and soils on land areas changed the biosphere by transforming the terrestrial environment and linking it more closely with the aquatic realm. This influenced the evolution of tetrapods and their colonization of the land. The global climate switched from the hot greenhouse phase of the earlier Palaeozoic to the cold icehouse phase of the later Palaeozoic. This change was associated with one of the most important extinction phases of the Earth's history (the late-Frasnian biodiversity crisis). The greenhouse climate favoured the development of reef ecosystems dominated by rugose corals and stromatoporoids. They reached their acme in the Middle Devonian and disappeared in the late Frasnian, probably due to the global expansion of black, anoxic sediments on the shelves. Recent data suggest that the Frasnian crisis was triggered in large part by pulses of global

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