The Devonian is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, England, where rocks from this period were first studied.

The first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. The arthropod groups of myriapods, arachnids and hexapods also became well-established early in this period, after starting their expansion to land at least from the Ordovician period.

Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the Age of Fishes. The placoderms began dominating almost every known aquatic environment. The ancestors of all four-limbed vertebrates (tetrapods) began adapting to walk on land, as their strong pectoral and pelvic fins gradually evolved into legs, though they were not fully established until the Late Carboniferous. In the oceans, primitive sharks became more numerous than in the Silurian and Late Ordovician.

The first ammonites, a subclass of molluscs, appeared. Trilobites, the mollusc-like brachiopods, and the great coral reefs were still common. The Late Devonian extinction which started about 375 million years ago severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order Proetida.

Devonian palaeogeography was dominated by the supercontinent of Gondwana to the south, the small continent of Siberia to the north, and the medium-sized continent of Laurussia to the east. Major tectonic events include the closure of the Rheic Ocean, the separation of South China from Gondwana, and the resulting expansion of the Paleo-Tethys Ocean. The Devonian experienced several major mountain-building events as Laurussia and Gondwana approached; these include the Acadian Orogeny in North America and the beginning of the Variscan Orogeny in Europe. These early collisions preceded the formation of Pangaea in the Late Paleozoic.

The Devonian seas

The Devonian seas were dominated by brachiopods, such as the spiriferids, and by tabulate and rugose corals, which built large 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, jawless, armored fish are common by the Lower Devonian. These early fish include a number of different groups. By the the Middle 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 sarcopterygians, the lobe-finned fish, which eventually produced the first tetrapods just before the end of the Devonian.

The Devonian landscape

By the Devonian Period, colonization of the land was well underway. Before this time, there was no organic accumulation in the soils, resulting in soils with a reddish color. This is indicative of the underdeveloped landscape, probably colonized only by bacterial and algal mats.

By the start of the Devonian, early terrestrial vegetation had begun to spread. These plants did not have roots or leaves like most plants today, and many had no vascular tissue at all. They probably spread vegetatively, rather than by spores or seeds, 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 grew quite tall. The progymnosperm Archaeopteris (see photo above) was a large tree with true wood. It was the oldest known tree until the 2007 identification of Wattieza in 2007. 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

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 shallow seas 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. Whatever the cause, it was about this time that the first vertebrates moved onto the land.

The Complete History of the Earth: Devonian Period :fishe:

When Fish Wore Armor :smoking-fish:

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