Devonian Period

Devonian Period, in geologic time, an interval of the Paleozoic Era that follows the Silurian Period and precedes the Carboniferous Period, spanning between about 419.2 million and 358.9 million years ago. The Devonian Period is sometimes called the “Age of Fishes” because of the diverse, abundant, and, in some cases, bizarre types of these creatures that swam Devonian seas. Forests and the coiled shell-bearing marine organisms known as ammonites first appeared early in the Devonian. Late in the period the first four-legged amphibians appeared, indicating the colonization of land by vertebrates.

geologic time

During most of the Devonian Period, North America, Greenland, and Europe were united into a single Northern Hemisphere landmass, a minor supercontinent called Laurussia or Euramerica. This union of the paleocontinents of Laurentia (comprising much of North America, Greenland, northwestern Ireland, Scotland, and the Chukotsk Peninsula of northeastern Russia) and Baltica (now most of northern Europe and Scandinavia) occurred near the beginning of the Devonian Period. Extensive terrestrial deposits known as the Old Red Sandstone covered much of its northern area, while widespread marine deposits 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 percent 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 name of the Devonian Period is derived from the county of Devon, Eng. The English geologist Adam Sedgwick and the Scottish geologist Roderick Murchison proposed the designation in 1839 for the marine rocks they encountered in southwestern England, following the recognition by another British geologist, William Lonsdale, that fossil corals from Torquay in Devon seemed intermediate in type between those of the Silurian System below and those of the Lower Carboniferous System above. This led to the conclusion that the fossil corals were marine equivalents of the terrestrial Old Red Sandstone rocks already known in Wales and Scotland. The recognition that such major paleogeographic differences existed was a great scientific advance, and it was soon confirmed when Sedgwick and Murchison visited Germany and again when Murchison discovered an intercalation of Devonian marine fossils and Old Red Sandstone fishes near St. Petersburg, Russia. By 1843 American geologist and paleontologist James Hall was able to describe equivalent rocks in eastern North America, but precise correlation with European rocks was not achieved until some years later.