From Simple English Wikipedia, the free encyclopedia
TheDevonian is the fourth period of thePaleozoicera and thePhanerozoiceon. It lasted from about 419 million years ago (mya) to about 359 mya.[1] It is named afterDevonshire,England, where rocks from this period were first studied.
Sea levels were high, and there was a great variety of fish and other marine organisms.[2] The Devonian fish included theAgnatha (jawless fish), theAcanthodii (spiny fish), thePlacoderms (armoured fish), theChondrichthyes (cartilaginous fish), and the earlyOsteichthyes (bony fish). The Devonianstrata are divided into lower, middle and upper subdivisions.
The supercontinent ofGondwana was to the south, and the continent ofSiberia to the north. The early form of the small continent ofEuramerica was in between. The supercontinentPangaea was just starting to form.
By the late Devonian, the land had been colonized by plants andinsects. In the oceans, massive reefs were built. Euramerica and Gondwana were beginning to converge into what would becomePangaea.
Tectonic andvolcanic activity was high. A series ofextinction events occurred towards the end of the Devonian, including a major event at the Frasnian-Famennian boundary in the Late Devonian, about 364 mya.
A major extinction occurred at the start of the last phase of the Devonian period, the Frasnian-Famennian boundary, about 372.2 mya. Almost all the fossilagnathan fishes suddenly disappeared. A second strong extinction closed the Devonian period. The late Devonian extinction was one of five major extinction events in the history of the Earth. The cause or causes are not known.[4]
