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

The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 439 million years before the present (BP), to the beginning of the Devonian period, about 408.5 million years BP. As with 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. See Ordovician-Silurian extinction events.

The 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 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 subdivisions

The 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:

• 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 a different series of regional stages is used:

• 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 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 fauna

Silurian 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 lycopods, and myriapods 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

• Paleos: (http://www.palaeos.com/Paleozoic/Silurian/Silurian.htm#history) Silurian
• UCMP Berkeley: (http://www.ucmp.berkeley.edu/silurian/silurian.html) The Silurian