The Devonian Period

All seven Devonian stages have been defined by Global Boundary Stratotype Sections and Points (GSSPs), but revisions of the base of the Emsian and of the Devonian–Carboniferous boundary are ongoing. Most of the Devonian Period was a time of exceptionally high sea-level stand and inferred widespread equable climates, but glaciations occurred immediately before its end in the south polar areas of Gondwana (South America, Central, and South Africa). There is even evidence for contemporaneous mountain glaciers in tropical latitudes (in the Appalachians of eastern North America). The cold-water Malvinocaffric Province of southern Gondwana existed throughout the Early Devonian but disappeared stepwise in the Middle Devonian. Most present-day continental areas and shelves were grouped in one hemisphere, creating a giant “Proto-Pacific” or Panthalassa Ocean, whose margins are poorly preserved in allochthonous terrains. Following the tectonic events of the Caledonian Orogeny of Laurasia, many “Old Red Sandstone” terrestrial deposits formed. After the closure of the narrow Rheic Ocean early in the Devonian, Eovariscan tectonic movements affected Middle and Late Devonian strata in the western Proto-Tethys of Europe and North Africa. Other active fold belts existed in western North America (early Antler Orogeny), polar Canada, in the Appalachians, in the Urals, along the southern margin of Siberia, in NW China, and in eastern Australia. The Devonian is the time of greatest carbonate production, with a peak of reef growth, and of the greatest diversity of marine fauna in the Paleozoic. Forests became established near the end of the Middle Devonian. Aquatic tetrapods appeared early in the Middle Devonian and diversified in the Upper Devonian.