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The Devonian Period: (408 million years ago)

 

The Devonian Period is named after Devon, in England, where rocks from this period were first studied.

 

During the Devonian Period, which occurred in the Paleozoic era, the first fish evolved legs and started to walk on land as tetrapods around 397 million years ago. Various terrestrial arthropods also became well-established.

 

The first seed-bearing plants spread across dry land, forming huge forests. In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician, and the first lobe-finned and ray finned (both are types of bony fish) evolved. The first ammonite mollusks appeared, and trilobites, the mollusc-like brachiopods, as well as great coral reefs were still common. The Late Devonian extinction severely affected marine life.

 

The paleogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small supercontinent of Euramerica in between.

 

The Devonian period was a time of great tectonic activity, as Euramerica and Gondwana drew closer together.

 

Near the equator, the plate of Euramerica and Gondwana were starting to meet, beginning the early stages of assembling Pangaea. This activity further raised the northern Appalachian Mountains and formed the Caledonian Mountains in Great Britain and Scandinavia.

 

Sea levels were high worldwide, and much of the land lay submerged under shallow seas, where tropical reef organisms lived. The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet.

 

 

Climate

 

The Devonian Period was a relatively warm period, and probably lacked any glaciers. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 °C (86 °F) in the Early Devonian. CO2 levels dropped steeply throughout the Devonian period as the burial of the newly-evolved forests drew carbon out of the atmosphere into sediments; this may be reflected by a Mid-Devonian cooling of around 5 °C (9 °F). The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in CO2 concentrations, continental weathering increases (as predicted by warmer temperatures); further, a range of evidence, such as plant distribution, points to Late Devonian warming.

 

 

The End of the Devenonian Period

A major extinction occurred at the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes, save for the psammosteid heterostracans, suddenly disappeared. A second strong pulse closed the Devonian period. The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota, more drastic than the familiar extinction event that closed the Cretaceous.

The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems .

 

Map of the Earth during the Devonian Period

 

 

 

 

 

 

 

 

Paleozoic Era

Mesozoic Era