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The Devonian is a major division of the geologic timescale that extends from the end of the Silurian period (360 million years ago (mya)) to the beginning of the Mississippian subperiod of the Carboniferous (408.5 mya). The Devonian period is named for England's Devonshire area where Devonian outcrops are common.
As with most older geologic periods, the rock beds that define period's the start and end are well identified, but the exact dates are uncertain by 5-15 million years.
Subdivisions
The Devonian is usually broken into lower, middle, and upper subdivisions. The Faunal stages from youngest to oldest are:
Upper (most recent) - Famennian/Chautauquan/Canadaway/Conneaut/Conneautan/Conewango/Conewangan
- Frasnian/Senecan/Sonyea/Sonyean/West Falls
Middle - Cazenovia/Cazenovian
- Givetian/Erian/Senecan/Tioughniogan/Tioughnioga/Taghanic/Taghanican/Genesee/Geneseean
- Eifelian/Southwood
Lower (oldest) - Helderberg
- Emsian/Sawkill/Deer Park
- Pragian/Siegenian
- Lochkovian/Gedinnian
Devonian rocks are oil and gas producers in some areas.
Palaeogeography The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer together. Near the equator, Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern Appalachian Mountains and forming the Caledonides in Britain and Scandinavia. The southern continents remained tied together in the supercontinent of Gondwana. The remainder of modern Eurasia lay in the Northern Hemisphere. Sea levels were high worldwide, and much of the land lay submerged under shallow seas, where tropical reef organisms lived. A deep, enormous ocean covered the rest of the planet.
The Devonian has been referred to as the "greenhouse age". Fossil signatures of widespread reefs indicate that the climate was mild and warm, as well as moderately humid. The Devonian has also been referred to as the "Age of Fish".
Fauna Fossil trilobite Ductina vietnamica from the Devonian of China
Marine biota Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods and corals. Lily-like crinoids were abundant, and trilobites were still fairly common, but less diverse than in earlier periods. The ostracoderms were joined in the mid-Devonian by the first jawed fishes, the great armored placoderms, as well as the first sharks and ray-finned fish. The first shark, the Cladoselache, appeared in the oceans during the Devonion period. They became abundant and diverse. In the late Devonian the lobe-finned fish appeared, giving rise to the first tetrapods.
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, which are constructed mainly by corals and calcareous algae. They were composed of calcareous algae and coral-like stromatoporoids, and tabulate and rugose corals, in that order of importance.
Terrestrial biota By the Devonian Period, life was well underway in its colonization of the land. The bacterial and algal mats were joined early in the period by primitive plants that created the first recognizable soils and harbored some arthropods like mites, scorpions and myriapods. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall.
By the late Devonian, forests of small, primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many were quite tall. The tree-like ancestral fern Archaeopteris, grew as a large tree with true wood. These are the oldest known trees of 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. This may have cooled the climate and led to a massive extinction event. see Late Devonian extinction.
Also in the Devonian, both vertebrates and arthropods were solidly established on the land.
See also
External links - UC Berkeley site introduces the Devonian. (http://www.ucmp.berkeley.edu/devonian/devlife.html)
- 'Devonian Times;' life and ecology. (http://www.mdgekko.com/devonian/index.html)
- Devonian reef system in northwest Australia. (http://www.nd.edu/~cneal/al375/Kimb_Geology.html)
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