The Devonian period is a geologic time interval within the Paleozoic Era spanning from the end of the Silurian Period, aapproximately 417 million years before present (BP), to the start of the Carboniferous, around 354 million years BP. The Devonian period is named for Devonshire, England.
The Rhynie Chert in Scotland is a Devonian age deposit containing fossils of both Zosterophyllophytes and Trimerophytes, the two major lines of vascular plants. This indicates that prior to the start of the Devonian, the first major radiations of the plants had already happened. The oldest known vascular plants in the Northern Hemisphere are Devonian.
The vegetation of the early Devonian consisted primarily of small plants, the tallest being only a meter tall. By the end of the Devonian, ferns, horsetails and seed plants had also appeared, producing the first trees and the first forests. Archaeopteris, shown below left, is one of these first trees.
Also during the Devonian, two major animal groups colonized the land. The first tetrapods, or land-living vertebrates, appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids. In the oceans, brachiopods flourished, like the beautifully pyritized brachiopod Paraspirifer bownockeri from Ohio, pictured above and to the right. Crinoids and other echinoderms, tabulate and rugose corals, and ammonites were also common. Many new kinds of fish appeared.
During the Devonian, there were three major continental masses: North America and Europe sat together near the equator, much of their current land underneath seas. To the north lay a portion of modern Siberia. A composite continent of South America, Africa, Antarctica, India, and Australia dominated the southern hemisphere.
The Devonian seas were dominated by brachiopods, such as the spiriferids, and by tabulate and rugose corals, which built large bioherms, or reefs, in shallow waters. Encrusting red algae also contributed to reef building. In the Lower Devonian, ammonoids appeared, leaving us large limestone deposits from their shells. Bivalves, crinoid and blastoid echinoderms, graptolites, and trilobites were all present, though most groups of trilobites disappeared by the close of the Devonian.
The Devonian is also notable for the rapid diversification in fish. Benthic armored fish are common by the Early Devonian. These early fish are collectively called "ostracoderms", and include a number of different groups. By the Mid-Devonian, placoderms, the first jawed fish, appear. Many of these grew to large sizes and were fearsome predators. Of the greatest interest to us is the rise of the first sarcopterygiians, or the lobe-finned fish, which eventually produced the first tetrapods just before the end of the Devonian.
By the Devonian Period, life was well underway in its colonization of the land. Before this time, there is no organic accumulation in the soils, causing these soil deposits to be a reddish color. This is indicative of the underdeveloped landscape, probably colonized only by bacterial and algal mats.
By the start of the Devonian, however, early terrestrial vegetation had begun to spread. These 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. These plants included the now extinct zosterophylls and trimerophytes. The early fauna living among these plants were primarily arthropods: mites, trigonotarbids, wingless insects, and myriapods, though these early faunas are not well known.
By the Late Devonian, lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many are rather tall plants. The progymnosperm Archaeopteris, whose leaves are shown at right, was a large tree with true wood. In fact it is the oldest such tree known, and produced some of the world's first forests. By the end of the Devonian, the first seed plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". Along with this diversification in terrestrial vegetation structure, came a diversification of the arthropods.
Tectonics and paleoclimate of the Devonian
Significant changes in the world's geography took place during the Devonian. During this period, the world's land was collected into two supercontinents, Gondwana and Euramerica. These vast landmasses lay relatively near each other in a single hemisphere, while a vast ocean covered the rest of the globe. These supercontinents were surrounded on all sides by subduction zones. With the development of the subduction zone between Gondwana and Euramerica, a major collision was set in motion that would bring the two together to form the single world-continent Pangea in the Permian.
In addition to global patterns of change, many important regional activities also occurred. The continents of North America and Europe collided, resulting in massive granite intrusions and the raising of the Appalachian Mountains of eastern North America. Vigorous erosion of these newly uplifted mountains yielded great volumes of sediment, which were deposited in vast lowlands and shallow seas nearby.
Extensive reef building, producing some of the world's largest reef complexes, proceeded as stromatoporoids and corals appeared in increasing numbers. These were built in the equatorial seas between the continents. Large areas of shallow sea in North America, central Asia, and Australia became basins in which great quantities of rock salt, gypsum, and other minerals precipitated.
Near the end of the Devonian, a mass extinction event occurred. Glaciation and the lowering of the global sea level may have triggered this crisis, since the evidence suggests warm water marine species were most affected. Meteorite impacts have also been blamed for the mass extinction, or changes in atmospheric carbon dioxide. It is even conceivable that it was the evolution and spread of forests and the first plants with complex root systems that may have altered the global climate. In any case, it was about this time that the first vertebrates moved onto the land.
Sources and further reading
- R. Goldring. Devonian, in The Ecology of Fossils, W.S.McKerrow, ed. 1978. MIT Press, Cambridge.Further reading
- W.J.Barclay, W.J. 1989. Geology of the South Wales Coalfield Pt II, the country around Abergavenny, 3rd edn. Memoir of the British Geological Survey Sheet 232 (Eng & Wales) pp18-19
- S.F.Parry, S.R.Noble, Q.G.Crowley & C.H.Wellman. 2011. "A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications". Journal of the Geological Society (London: Geological Society) 168 (4): 863–872.
- B.Kaufmann, E.Trapp and K.Mezger. 2004. "The numerical age of the Upper Frasnian(Upper Devonian) Kellwasser horizons: A new U-Pb zircon date from Steinbruch Schmidt(Kellerwald, Germany)". The Journal of geology 112 (4): 495–501
- Geologic Time pages updated to reflect Geological Society of America (GSA), 1999. Geologic Timescale, compiled by A.R. Palmer and J. Geissman
- University of California Museum of Paleontology Homepage
The basis of this article was developed by Brian R. Speer, Rachel Grande, Larry Jean, Aariel Rowan, Angela Sumner and Amy Tang. Editors from the Encyclopedia of Earth have expanded and modified the original text produced by those authors.