Dinopedia
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System (period) Series Stage (age) Lower boundary, Ma
Devonian Lower Lochkovian 419.2±3.2
Silurian Pridoli Golden spike423.0±2.3
Ludlow Ludfordian Golden spike425.6±0.9
Gorstian Golden spike427.4±0.5
Wenlock Homerian Golden spike430.5±0.7
Sheinwoodian Golden spike433.4±0.8
Llandovery Telychian Golden spike438.5±1.1
Aeronian Golden spike440.8±1.2
Rhuddanian Golden spike443.8±1.5
Ordovician Upper Hirnantian older
Subdivisions and "golden spikes" according to IUGS as of September 2023[1]
The silurian sea by zdenek burian 1951

The Silurian is the third period (system in stratigraphy) of the Paleozoic era. It extends from the end of the Hirnantian age of the Ordovician period, about 443.8 ± 1.5 million years ago, to the beginning of the Lochkovian age of the Devonian period, about 419.2 ± 3.2 Ma (IUGS 2023). All stages of the Silurian have the GSSPs. The base of the Silurian is set at the Ordovician-Silurian extinction event when 60% of marine species were wiped out.

Silurianfishes ntm 1905 smit 1929

In the Silurian first undoubted land animals appeared. The scorpion Palaeophonus and the myriapod Pneumodesmus existed in the Wenlock epoch.

Historiography[]

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 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 the friendship. Charles Lapworth eventually resolved the conflict by defining a new Ordovician system including the contended beds.

Silurian subdivisions[]

The Silurian Period of time is usually broken into early (Llandovery and Wenlock) and late (Ludlow and Pridoli) subdivisions (epochs). Nevertheless, some schemes use an early (Llandovery), middle (Wenlock) and late (Ludlow and Pridoli) breakdown. These faunal stages are characterized by their index fossils, new species of colonial marine Graptolites that appeared in each. Epochs of time correspond to series of rocks (as periods of time correspond to systems of rocks), which are referred to as belonging to the lower, middle, or upper part of the rock column, analogous to early, middle, or late Silurian time. The epochs and stages from youngest to oldest are:

  • Pridoli Epoch - no stages defined (late Silurian)
  • Ludlow Epoch divided into
    • Ludfordian (late Ludlow - late Silurian)
    • Gorstian (early Ludlow - late Silurian)
  • Wenlock Epoch divided into
    • Homerian (late Wenlock - early or middle Silurian)
    • Sheinwoodian (early Wenlock - early or middle Silurian)
  • Llandovery Epoch divided into
    • Telychian (late Llandovery - early Silurian)
    • Aeronian (mid Llandovery - early Silurian)
    • Rhuddanian (early Llandovery - early Silurian)

In North America a different suite of regional stages is used:

  • Cayugan (Late Silurian - Ludlow)
  • Lockportian (Middle Silurian - Wenlock)
  • Tonawandan (Middle Silurian - Wenlock)
  • Ontarian (Early Silurian - Llandovery)
  • Alexandrian (Early Silurian - Llandovery)

Silurian paleogeography[]

Silurian

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, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second Supercontinent known as Euramerica.

When the proto-Europe collided with North America, the collision folded coastal sediments that had been accumulating since the Cambrian off the east coast of North America and the west coast of Europe. This event is the Caledonian orogeny, a spate of mountain building that stretched from New York State through conjoined Europe and Greenland to Norway. At the end of the Silurian, sea levels dropped again, leaving telltale basins of evaporites in a basin extending from Michigan to West Virginia, and the new mountain ranges were rapidly eroded. The Teays River, flowing into the shallow mid-continental sea, eroded Ordovician strata, leaving traces in the Silurian strata of northern Ohio and Indiana.

The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include, Proto-Tethys, Paleo-Tethys, Rheic Ocean, a seaway of Iapetus Ocean (now in between Avalonia and Laurentia), and newly formed Ural Ocean.

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. Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.

Gallery[]

References[]

Further reading[]

  • Emiliani, Cesare, 1993. Planet Earth : Cosmology, Geology and the Evolution of Life and Environment.
  • Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

External links[]

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