Silurian

Alternate use: The Silurians, a reptillian race from the science fiction series "Doctor Who".

The Silurian is a major division of the geologic timescale that extends from about 408.5 million years before the present (BP) with the end of the Ordovician period until 443.5 million years BP with the beginning of the Devonian period. As with most other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by 5-10 million years. 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.

Silurian subdivisions

The Silurian is usually broken into lower (Llandovery and Wenlock) and upper (Ludlow and Pridoli) subdivisions. Still, some schemes, 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)

Silurian paleogeography

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 melting of icecaps and glaciers contributed to a rise in sea level. The other continents drifted together near the equator, starting the formation of a second supercontinent known as Laurasia.

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 fauna

Life was abundant in the Silurian. The 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 tissue that carries food, appeared in the Silurian period. Rhyniophytes, primitive lycopods, and myriapods became the first proper terrestrial organisms. Adding to the terrestrial ecosystem were relatives of modern spiders and centipedes.

cy:Silwraidd de:Silur (Geologie) fr:Silurien nl:Siluur pl:Sylur zh:志留纪