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DEVONIAN PERIOD

416-359 million years ago

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By the end of the Devonian, forest were spreading, oxygen levels were rising, and landmasses near the equator were becoming increasingly terrestrial as the two supercontinents joined, mountain ranges began, and sea levels dropped.

At the beginning of the Devonian, atmospheric oxygen concentrations were less than half those of modern times. During the Devonian, oxygen levels increased increased and carbon dioxide decreased (perhaps by half). At the beginning of the Devonian, the Iapetus Ocean separated Laurussia (the Northern continent which included North America, Europe, and most of Asia) and Gondwana (which included South America, Africa, Australia, Antarctica, and India). During the Devonian, these supercontinents began to join, creating mountain ranges and changing weather patterns. The lands near the equator (such as Greenland, North America, and Australia where the earliest amphibian fossils are known) became increasingly terrestrial as the ocean receded and mountain ranges began to form (such as the Appalachians). Glaciations which affected Gondwana resulted in a large south polar ice cap which lowered ocean levels. Although there were land plants in the Early Devonian, they did not yet form dense forests and therefore offered less protection from flooding. The ancestors of amphibians lived in a world in which water levels could rise and fall very quickly after rainfall. The ability of rhipidistian fish to breathe and move both on land and water undoubtedly helped them adapt to this instability. By the Mid-Devonian, shrublike, bushy plants, had evolved and forests of Archaeopteris were common. Land invertebrates thrived in these new environments, although insects probably did not extensively adapt to feeding on plants until the Carboniferous (Clack, 2002).