In the Lower Devonian, a secondary supercontinent to Gondwanaland was formed by the collision of Laurentia and Baltica; amongst its repercussions were the raising of the Appalachian and Caledonian Mountains. The two early supercontinents were encircled by subduction zones that would lead, eventually, to the creation of the supercontinent of Pangaea in the Permian period. The Upper Devonian atmosphere had carbon dioxide levels approximately eight times greater than those of today; oxygen levels stood at about 75 per cent of modern readings. The climate was uniformly warm and dry at the start of the period, but there was appreciable cooling in the Middle Devonian. In the warm seas, which covered 85 per cent of earth’s surface area, a vast profusion of fish species flourished, with armoured and jawless varieties still in the ascendant. Ammonites appeared for the first time, while trilobites were in decline.
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