DEFINITION OF DEVONIAN

The fourth geological period in the Paleozoic

Era, ranging from 408 million to 360 million years ago. It is

named after exposures in Devonshire in southwest England

and was first described in detail by Adam Sedgwick and Roderick

I. Murchison in 1839. The Devonian is divided into

three series and seven stages based on its marine fauna.

Devonian rocks are known from all continents and

reflect the distribution of the continents grouped into a large

remnant Gondwanan fragment in the Southern Hemisphere,

and parts of Laurasia (North America and Europe), Angaraland

(Siberia), China, and Kazakhstania in the Northern

Hemisphere. The eastern coast of North America and adjacent

Europe experienced the Acadian orogeny, formed in

response to subduction and eventual collision between Avalonian

fragments and ultimately Africa with Laurasia. Other

orogenies affected North China, Kazakhstania, and other

fragments. These mountain building events shed large clastic

wedges, including the Catskill delta in North America and

the Old Red sandstone in the British Isles.

The Devonian experienced several global (eustatic) sea

level changes and had times of glaciation. There appeared to

be a strong climatic gradation with tropical and monsoonal

conditions in equatorial regions and cold water conditions in

more polar regions.

Marine life in the Devonian was prolific, with brachiopods

reaching their peak. Rugose and tabulate corals,

stromatoporoids, and algae built carbonate reefs in many

parts of the world, including North America, China, Europe,

North Africa, and Australia. Crinoids, trilobites, ostracods,

and a variety of bivalves lived around the reefs and in other

shallow water environments, whereas calcareous foraminifera

and large ammonites proliferated in the pelagic realm. The

pelagic conodonts peaked in the Devonian, and their great

variety, widespread distribution, and rapid changes make

them useful biostratigraphic markers and form the basis for

much of the biostratigraphic division of Devonian time. Bony

fish evolved in the Devonian and evolved into tetrapod

amphibia by the end of the period.

The land was inhabited by primitive plants in the Early

Devonian, but by the middle of the period great swampy

forests with giant fern trees (Archaeopteris) and spore-bearing

organs populated the land. Insects, including some flying

varieties, were found in these swamps.

The end of the Devonian saw widespread mass extinction

of some marine animal communities, including brachiopods,

trilobites, conodonts and corals. The cause of this

extinction is not well known, with models including cooling

caused by a southern glaciation, or a meteorite impact.

See also APPALACHIANS; PALEOZOIC.

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