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Devonian



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The Devonian is the fourth period of the Paleozoic era and the Phanerozoic eon. It lasted from about 419 million years ago (mya) to about 359 mya.[1] It is named after Devonshire, 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 the Agnatha (jawless fish), the Acanthodii (spiny fish), the Placoderms (armoured fish), the Chondrichthyes (cartilaginous fish), and the early Osteichthyes (bony fish). The Devonian strata are divided into lower, middle and upper subdivisions.

During the Devonian the evolution of fish into tetrapods occurred.[3] Various terrestrial arthropods also became established. The first seed-bearing plants spread across dry land, forming huge forests.

The supercontinent of Gondwana was to the south, and the continent of Siberia to the north. The early form of the small continent of Euramerica was in between. The supercontinent Pangaea was just starting to form.

By the late Devonian, the land had been colonized by plants and insects. In the oceans, massive reefs were built. Euramerica and Gondwana were beginning to converge into what would become Pangaea.

Period ended with extinctions

Tectonic and volcanic activity was high. A series of extinction 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 fossil agnathan 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]

References

  1. Gradstein, Felix M; Ogg J.G. & Smith A.G. 2004. A geologic time scale. Cambridge: Cambridge University Press.

    Template-specific style sheet:

    ISBN 0-521-78673-8.
  2. The Devonian is sometimes called the Age of Fish.
  3. see Tetrapod#fishapods for discussion and references.
  4. McGhee, George R. Jr, 1996. The late Devonian mass extinction: the Frasnian/Famennian crisis. Columbia University Press. ISBN 0-231-07504-9