The devonian time period
editThe Devonian Period 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 is a geologic period of the Paleozoic era spanning from 416±2.8 to 359.2±2.5 million years ago (ICS, 2004)[1]. It is named after Devon, England, where Exmoor rocks from this period were first studied. During the Devonian Period, which occurred in the Paleozoic era, the first fish evolved legs[2]and started to walk on land as tetrapods around 365 Ma, and the first insects and spiders also started to colonize terrestrial habitats. 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 bony fish. 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 the middle. [edit] Devonian subdivisions The Devonian Period is formally broken into Early, Middle, and Late subdivisions. The rocks corresponding to these epochs are referred to as belonging to the Lower, Middle, and poopy heads Upper parts of the Devonian System. The international standard faunal stages from youngest to oldest are: [edit] Late (most recent) • Famennian • Frasnian [edit] Middle • Givetian • Eifelian [edit] Early (oldest) • Emsian • Pragian • Lochkovian Devonian palaeogeography The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer together. The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the Tropic of Capricorn, which is formed as much in Paleozoic times as nowadays by the convergence of two great airmasses, the Hadley cell and the Ferrel cell. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidized iron (hematite) characteristic of drought conditions. Near the equator, Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern Appalachian Mountains and forming the Caledonian Mountains in Great Britain and Scandinavia. The west coast of Devonian North America, by contrast, was a passive margin with deep silty embayments, river deltas and estuaries, in today's Idaho and Nevada; an approaching volcanic island arc reached the steep slope of the continental shelf in Late Devonian times and began to uplift deep water deposits, a collision that was the prelude to the mountain-building episode of Mississippian times called the Antler orogeny [1]. 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. The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys, Rheic Ocean, and Ural Ocean (which was closed during the collision with Siberia and Baltica). Devonian rocks are oil and gas producers in some areas.
some of this information was aquired from wikipedia pages.
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