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Devonian
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Devonian

This period is part of the
Paleozoic era.
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian

The Devonian is a geologic period that extends from about 360 to 408.5 million years before the present.

As with most older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the end of the period is uncertain by 5-15 million years.

The Devonian is named for England's Devonshire area where Devonian outcrops are common. The Devonian follows the Silurian period and precedes the Mississippian subperiod of the Carboniferous.

Table of contents
1 Devonian subdivisions
2 Devonian palaeogeography
3 Devonian fauna
4 External links

Devonian subdivisions

The Devonian is usually broken into Lower, Middle, and Upper subdivisions. The Faunal stages from youngest to oldest are:

Devonian rocks are oil and gas producers in some areas.

Devonian palaeogeography

The southern continents remained tied together into the Supercontinent Gondwana. In equatorial regions, North America and Europe formed a continent. The remainder of modern Eurasia lay in the Northern Hemisphere.

During this time period, the subcontinent of Euramerica was home to all of the new things that the Devonian brought. Sea levels were high world wide. Much of the land lay under underneath shallow seas, where tropical reef organisms lived. A huge, deep ocean covered the rest of the planet

The Devonian was called the “greenhouse age.” Widespread reefs indicated that the climate was mild and warm, as well as generally dry.

Devonian fauna

Marine biota

Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by
bryozoa, diverse and abundant brachiopods and corals. Trilobites were still fairly common, but less diverse than in earlier periods. The great armored fishes called placoderms were joined in the mid-Devonian by the first fishes with jaws. They became abundant and diverse. The first sharks appeared early in the period. Bony fish, many of substantial size appeared shortly thereafter. In the Late Devonian our ancestors, the lobe-finned bony fishes appeared, giving rise to the first tetrapods to crawl out on land at the end of the period.

Devonian reefs

A great barrier reef, now left high and dry in the Kimberley Basin of northwest Australia, once extended a thousand kilometers, fringing a Devonian continent. Reefs in general are built by various carbonate-secreting organisms that have the ability to erect wave-resistant frameworks close to sea level. The main contributors of the Devonian reefs were unlike modern reefs that are constructed by corals and calcareous algae: calcareous algae and coral-like stromatoporoids, and tabulate and rugose corals, in that order of importance.

Terrestrial biota

On land, the bacterial and algal mats were joined early in the period by low spreading primitive plants that created the first recognizable soils and harbored some arthropods like mites and scorpions and myriapods. By the Late Devonian, forests of primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many are rather tall plants. The tree-like ancestral fern Archaeopteris, grew as a large tree with true wood, the oldest known trees in the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". The primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterizes a recognizably modern world had its genesis in the late Devonian.

The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped, cooling the climate and leading to a massive extinction event.

Both vertebrates and arthropods were solidly established on the land.

External links