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The Silurian is a Geologic Period that extends from about 408.5 to 443.5 million years before the present. As with most other geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by 5-10 million years. The Silurian period follows the Ordovician and is followed by the Devonian period. The base of the Silurian is set at a major extinction event where 60% of marine species were wiped out.
The Silurian system was first described by Sir Roderick Murchison in the 1830s based on rocks in South Wales. It is named for a Welsh Celtic tribe -- the Silures. The series quickly came to overlap Adam Sedgwick's Cambrian sequence. Lapworth eventually resolved the conflict by defining a new Ordovician system including the contended beds.
The Silurian is usually broken into Lower (Llandovery and Wenlock) and Upper (Ludlow and Pridoli) subdivisions. Some workers, use a Lower (Llandovery), Middle (Wenlock) and Upper (Ludlow and Pridoli) breakdown. The series and stages from youngest to oldest are:
- Pridoli Series - no stages defined (Upper Silurian)
- Ludlow Series divided into
- Ludfordian (late Ludlow - Upper Silurian)
- Gorstian (early Ludlow - Upper Silurian)
- Wenlock Series divided into
- Homerian (late Wenlock - Lower Silurian
- Sheinwoodian (early Wenlock - Lower Silurian
- Llandovery Series divided into
- Telychian (late Llandovery - Lower Silurian)
- Aeronian (mid Llandovery - Lower Silurian)
- Rhuddanian (early Llandovery - Lower Silurian)
In North America the regional stages used are Cayugan (Late Silurian - Ludlow) Lockportian (Middle Silurian - Wenlock) Tonawandan (Middle Silurian - Wenlock) Ontarian (Lower Silurian - Llandovery) Alexandrian (Lower Silurian - Llandovery)
During the Silurian, Gondwana remained in high Southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician. The other continents drifted together near the equator starting the formation of a second supercontinent known as Laurasia.
Life was abundant in the Silurian. 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 colonial economy.
The Silurian was a time of high sea levels in Eastern North America and Europe. Primitive multicelled land plants are found from the Silurian. 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. Brachiopods, bryozoa, mollusks, and trilobites were abundant and diverse