 |
 |
|
 |
 |
|
|
|
|
|
|
Encyclopedia :
S :
SI :
SIL :
Silurian |
|
|
Silurian
OriginThe Silurian system was first identified by Sir Roderick Murchison, who was examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s. He named the sequences for a Celtic tribe of Wales— the Silures, extending the convention his friend Adam Sedgwick had established for the "cambrian". In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, which was the germ of the modern geological time scale. The "Silurian" series quickly came to overlap Sedgwick's Cambrian sequence, provoking furious disagreements that ended the friendship. Charles Lapworth eventually resolved the conflict by defining a new Ordovician system including the contended beds. Silurian subdivisionsThe Silurian is usually broken into lower (Llandovery and Wenlock) and upper (Ludlow and Pridoli) subdivisions (epochs). Nevertheless, some schemes use a lower (Llandovery), middle (Wenlock) and upper (Ludlow and Pridoli) breakdown. These faunal stages are characterized by their index fossils, new species of colonial marine Graptolites that appeared in each. The series and stages from youngest to oldest are:
Silurian paleogeographyDuring the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician glaciation. The melting of icecaps and glaciers contributed to a rise in sea level. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Laurasia. This is the time of the Caledonian orogeny, a spate of mountainbuilding in conjoined Europe and Greenland. During this period, the Earth entered a long warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. The period witnessed a relative stabilization of the Earth's general climate, ending the previous pattern of erratic climatic fluctuations. Silurian faunaSilurian high sea levels and warm shallow continental seas provided a hospitable environment for marine life of all kinds. Silurian beds are oil and gas producers in some areas. Extensive beds of Silurian hematite -- an iron ore -- in eastern North America were important to the early American colonial economy. Coral reefs made their first appearance during this time. A few arthropods seem to have invaded the land during the Silurian. Fish reached considerable diversity and developed movable jaws. A diverse fauna of Eurypterus (Sea Scorpions) -- some of them several meters in length -- graced the shallow Silurian seas of North America. Many of their fossils have been found in the state of New York. Brachiopods, bryozoa, mollusks, and trilobites were abundant and diverse. The first fossil records of vascular plants, that is, land plants with tissues that carry food, appeared in the Silurian period. Rhyniophytes, primitive lycopodss, and myriapodss became the first proper terrestrial organisms. The terrestrial ecosystems included the first multicellular animals that have been identified, relatives of modern spiders and centipedes whose fossils were discovered in the 1990s. External links
|
|
|
This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License. |
|
| © 2008 Chamas Enterprises Inc. |
