Silurian

the silurian

The Silurian system was first identified by British geologist Sir Roderick Impey 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, following the convention his friend Adam Sedgwick had established for the Cambrian. In 1835 the two men presented a joint paper, underthe 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. As it was first identified, the "Silurian" series when traced farther afield quickly came to overlap Sedgwick's "Cambrian" sequence, however, provoking furious disagreements that ended thefriendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland.
The French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge. He dividedthe Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, and the later stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils.
Subdivisions

LlandoverySee also: Llandovery Group
The Llandovery epoch lasted from 443.7 ± 1.5 to 428.2 ± 2.8, and is subdivided into three stages: the Rhuddanian, lasting until 439 million years ago, the Aeronian, lasting to 436 million years ago, and the Telychian. The epoch is named for the town of Llandovery in Carmarthenshire, Wales.
Wenlock
See also: Wenlock Group
The Wenlock, which lasted from 428.2 ± 1.5 to422.9 ± 2.8, is subdivided into the Sheinwoodian (to 426.2 million years ago) and Homerian ages. It is named after the Wenlock Edge in Shropshire, England. During the Wenlock, the oldest known tracheophytes of the genus Cooksonia, appear. The complexity of slightly younger Gondwana plants like Baragwanathia indicates either a much longer history for vascular plants, perhaps extending into theearly Silurian or even Ordovician. See Evolutionary history of plants.
Ludlow
See also: Ludlow Group
The Ludlow, lasting from 422.9 ± 1.5 to 418.7 ± 2.8, comprises the Gorstian stage, lasting until 421.3 million years ago, and the Ludfordian stage. It is named for the town of Ludlow in Shropshire, England.
Přídolí
The Pridoli, lasting from 418.7 ± 1.5 to 416 ± 2.8, is the final and shortestepoch of the Silurian. It is named after one locality at natural reserve Homolka a Přídolí near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area.[9]
Regional stages
In North America a different suite of regional stages is sometimes used:
• Cayugan (Late Silurian - Ludlow)
• Lockportian (middle Silurian: late Wenlock)
• Tonawandan (middleSilurian: early Wenlock)
• Ontarian (Early Silurian: late Llandovery)
• Alexandrian (earliest Silurian: early Llandovery)
Geography

Ordovician-Silurian boundary exposed on Hovedøya, Norway, showing the very marked difference between the light gray Ordovician calcareous sandstone and brown Silurian mudstone. The layers have been inverted (overturned) by the Caledonian orogeny.
With thesupercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.[10] The high sea levels of the Silurian and the relatively flat land (with few significant mountain belts) resulted in a number of island chains, and thus a rich diversity of environmental settings.[10]
During the Silurian, Gondwana continued a slow southward drift...