Devionian Period

Devonian Period When the Devonian period dawned about 416 million years ago the planet was changing its appearance. The great supercontinent of Gondwana was headed steadily northward, away from the South Pole, and a second supercontinent began to form that straddled the Equator. Known as Euramerica, or Laurussia, it was created by coming together of parts of North America, Northern Europe, Russia, and Greenland. Extensive terrestrial deposits known as the Old Red Sandstone covered much of its northern area, while widespread marine deposit accumulated on its southern portion. The paleoequator (the site of the Equator at a point in the geological past) passed through North America and through China, which was at that time a separate landmass. South America, Africa, India, Australia, and Antarctica were joined into the Southern Hemisphere continent of Gondwana. Parts of this continent were also often covered by seawater. An ocean covered approximately 85% of the Devonian globe. There is limited evidence of ice caps, and the climate is thought to have been warm and equitable. The oceans experienced episodes of reduced dissolved oxygen levels, which likely caused the extinction of many species, especially marine animals. These extinctions were followed by periods of species diversification, as the descendants of surviving organisms filled in abandoned habitats. The physical geography of the Devonian can be reconstructed using evidence from paleomagnetism, paleoclimate, paleobiogeography, and tectonic events. Because the paleomagnetic data for the Devonian is conflicting, recent efforts to describe the positions of the continents have concentrated on the rocks types associated with particular environments. Such methods focus on the distribution of evaporites, shelf carbonates, and corals because present-day deposits of these types have specific, well-known climatic constraints. Faunal Distributions are also employed but to a lesser extent.