Silurian

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Silurian
443.8 ± 1.5 – 419.2 ± 3.2 Ma
Period
Stratigraphic unitSystem
First proposed byRoderick Murchison, 1835
Time span formalityFormal
Lower boundary definitionFAD of the Graptolite Akidograptus ascensus
Lower boundary GSSPDob's Linn, Moffat, United Kingdom
55°26′24″N 3°16′12″W / 55.4400°N 3.2700°W / 55.4400; -3.2700
Lower GSSP ratified1984[4][5]
Upper boundary definitionFAD of the Graptolite Monograptus uniformis
Upper boundary GSSPKlonk, Czech Republic
49°51′18″N 13°47′31″E / 49.8550°N 13.7920°E / 49.8550; 13.7920
Upper GSSP ratified1972[6]
Atmospheric and climatic data
Sea level above present dayAround 180 m, with short-term negative excursions[7]

The Silurian (

Ordovician–Silurian extinction events
when up to 60% of marine genera were wiped out.

One important event in this period was the initial establishment of terrestrial life in what is known as the

arachnids and hexapods) became fully terrestrialized.[15]

A significant evolutionary milestone during the Silurian was the diversification of

History of study

The Silurian system was first identified by the Scottish geologist 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, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales.[17] This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures (cf. Geologic map of Wales, Map of pre-Roman tribes of Wales). 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.[18] 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.

The English geologist Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds.[19] An alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland.[20]

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 divided the Silurian rocks of Bohemia into eight stages.[21] His interpretation was questioned in 1854 by Edward Forbes,[22] 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 Silurian fossils.

Subdivisions

Subdivisions of the Silurian period
Epoch
Age
Start
(mya)
Etymology of
Epochs and Stages
Notes
Llandovery Rhuddanian 443.8 Cefn-Rhuddan Farm, Llandovery in Carmarthenshire, Wales
Aeronian 440.8 Cwm Coed-Aeron Farm, Wales Trefawr Track near the farm is the site of the GSSP
Telychian 438.5 Pen-lan-Telych Farm, Llandovery, Wales
Wenlock
Sheinwoodian 433.4 Sheinwood village, Much Wenlock and Wenlock Edge, Shropshire, England During the Wenlock, the oldest-known
millipedes from Scotland.[23]
Homerian 430.5 Homer, Shropshire, England
Ludlow Gorstian 427.4 Gorsty village near Ludlow, Shropshire, England
Ludfordian 425.6 Ludford, Shropshire, England
Přídolí
423.0 Named after a locality at the Homolka a Přídolí nature reserve near the Prague suburb of Slivenec, Czech Republic. Přídolí is the old name of a cadastral field area.[24]

Paleogeography

Ordovician-Silurian boundary on Hovedøya, Norway, showing brownish late Ordovician mudstone and later dark deep-water Silurian shale. The layers have been overturned by the Caledonian orogeny.

With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.[25] 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.[25]

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 southern continents remained united during this period. The melting of icecaps and

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 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 Period strata, forming deposits of Silurian strata in northern Ohio and Indiana.

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

Fossils of the late Silurian sea bed

Climate and sea level

The Silurian period was once believed to have enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the Ordovician before it and the extreme heat of the ensuing Devonian; however, it is now known that the global climate underwent many drastic fluctuations throughout the Silurian,[26][27] evidenced by numerous major carbon and oxygen isotope excursions during this geologic period.[28][29][30] Sea levels rose from their Hirnantian low throughout the first half of the Silurian; they subsequently fell throughout the rest of the period, although smaller scale patterns are superimposed on this general trend; fifteen high-stands (periods when sea levels were above the edge of the continental shelf) can be identified, and the highest Silurian sea level was probably around 140 metres (459 ft) higher than the lowest level reached.[25]

During this period, the Earth entered a warm greenhouse phase, supported by high CO2 levels of 4500 ppm, and warm shallow seas covered much of the equatorial land masses.[31] Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian.[27] Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.[32]

Perturbations

The climate and

tillites in the middle to late Silurian make this explanation problematic.[34]

Flora and fauna

The Silurian period has been viewed by some palaeontologists as an extended recovery interval following the Late Ordovician mass extinction (LOME), which interrupted the cascading increase in biodiversity that had continuously gone on throughout the Cambrian and most of the Ordovician.[35]

The Silurian was the first period to see megafossils of extensive terrestrial biota in the form of

nematophytes, heralding the beginning of the Silurian-Devonian Terrestrial Revolution.[12][13][36] However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian.[25]

The first fossil records of vascular plants, that is, land plants with tissues that carry water and food, appeared in the second half of the Silurian Period.[37] The earliest-known representatives of this group are Cooksonia. Most of the sediments containing Cooksonia are marine in nature. Preferred habitats were likely along rivers and streams. Baragwanathia appears to be almost as old, dating to the early Ludlow (420 million years) and has branching stems and needle-like leaves of 10–20 centimetres (3.9–7.9 in). The plant shows a high degree of development in relation to the age of its fossil remains. Fossils of this plant have been recorded in Australia,[38][39] Canada,[40] and China.[41] Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils[42] of Early Silurian (Llandovery) age.[43] The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae.[42]

Fossils that are considered as terrestrial animals are also known from the Silurian. The definitive oldest record of

Wenlock epoch of the middle Silurian.[51] Regardless, Pneumodesmus is still an important fossil as the oldest definitive evidence of spiracles to breath in the air.[44]

The first bony fish, the

Photosymbiotic scleractinians made their first appearance during the Middle Silurian.[76] Reef abundance was patchy; sometimes, fossils are frequent, but at other points, are virtually absent from the rock record.[25]

Notes

  1. .
  2. .
  3. ^ "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  4. .
  5. . Retrieved 11 December 2020.
  6. ^ Chlupáč, Ivo; Hladil, Jindrich (January 2000). "The global stratotype section and point of the Silurian-Devonian boundary". CFS Courier Forschungsinstitut Senckenberg. Retrieved 7 December 2020.
  7. S2CID 206514545
    .
  8. .
  9. ^ "Silurian". Dictionary.com Unabridged (Online). n.d.
  10. CollinsDictionary.com. HarperCollins
    .
  11. ^ "International Chronostratigraphic Chart v.2015/01" (PDF). International Commission on Stratigraphy. January 2015.
  12. ^
    S2CID 249616013
    .
  13. ^ . Retrieved 8 December 2022.
  14. .
  15. .
  16. .
  17. ^ See:
  18. ^ Sedgwick; Murchison, R.I. (1835). "On the Silurian and Cambrian systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales". Report of the Fifth Meeting of the British Association for the Advancement of Science. § Notices and Abstracts of Miscellaneous Communications to the Sections. 5: 59–61.
  19. S2CID 129165105
    . From pp. 13–14: "North Wales itself – at all events the whole of the great Bala district where Sedgwick first worked out the physical succession among the rocks of the intermediate or so-called Upper Cambrian or Lower Silurian system; and in all probability much of the Shelve and the Caradoc area, whence Murchison first published its distinctive fossils – lay within the territory of the Ordovices; … Here, then, have we the hint for the appropriate title for the central system of the Lower Palaeozoics. It should be called the Ordovician System, after this old British tribe."
  20. Albert Auguste Cochon de Lapparent (1839–1908): Lapparent, A. de (1893). Traité de Géologie
    (in French). Vol. 2 (3rd ed.). Paris, France: F. Savy. p. 748. From p. 748: "D'accord avec ces divisions, on distingue communément dans le silurien trois étages: l'étage inférieur ou cambrien (1); l'étage moyen ou ordovicien (2); l'étage supérieur ou gothlandien (3)." (In agreement with these divisions, one generally distinguishes, within the Silurian, three stages: the lower stage or Cambrian [1]; the middle stage or Ordovician [2]; the upper stage or Gotlandian [3].)
  21. ^ Barrande, Joachim (1852). Systême silurien du centre de la Bohême (in French). Paris, France and Prague, (Czech Republic): (Self-published). pp. ix–x.
  22. ^ Forbes, Edward (1854). "Anniversary Address of the President". Quarterly Journal of the Geological Society of London. 10: xxii–lxxxi. See p. xxxiv.
  23. ^ a b Paul Selden & Helen Read (2008). "The oldest land animals: Silurian millipedes from Scotland" (PDF). Bulletin of the British Myriapod & Isopod Group. 23: 36–37.
  24. .
  25. ^ .
  26. . Retrieved 8 December 2022.
  27. ^ .
  28. .
  29. . Retrieved 8 December 2022.
  30. ^ .
  31. .
  32. .
  33. .
  34. .
  35. .
  36. . Retrieved 23 November 2022.
  37. .
  38. . Retrieved 8 December 2022.
  39. .
  40. .
  41. ^ Bora, Lily (2010). Principles of Paleobotany. Mittal Publications. pp. 36–37.
  42. ^
    S2CID 21794174
    .
  43. , p. 4
  44. ^ .
  45. .
  46. ^ Shear, William A.; Jeram, Andrew J.; Selden, Paul (1998). Centiped legs (Arthropoda, Chilopoda, Scutigeromorpha) from the Silurian and Devonian of Britain and the Devonian of North America. New York, NY: American Museum of Natural History.
  47. PMID 17810866
    .
  48. .
  49. .
  50. .
  51. .
  52. . Retrieved 16 April 2023.
  53. . Retrieved 16 April 2023.
  54. . Retrieved 16 April 2023.
  55. ^ . Retrieved 23 November 2022.
  56. . Retrieved 16 April 2023.
  57. doi:10.1666/0094-8373(2003)0292.0.CO;2 (inactive 31 January 2024). Retrieved 16 April 2023.{{cite journal}}: CS1 maint: DOI inactive as of January 2024 (link
    )
  58. .
  59. .
  60. . Retrieved 16 April 2023.
  61. . Retrieved 16 April 2023.
  62. . Retrieved 16 April 2023.
  63. ^ . Retrieved 16 April 2023.
  64. . Retrieved 16 April 2023.
  65. . Retrieved 16 April 2023.
  66. . Retrieved 16 April 2023.
  67. . Retrieved 16 April 2023.
  68. doi:10.1666/0094-8373(2000)0262.0.CO;2 (inactive 31 January 2024). Retrieved 16 April 2023.{{cite journal}}: CS1 maint: DOI inactive as of January 2024 (link
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  69. . Retrieved 16 April 2023.
  70. .
  71. . Retrieved 16 April 2023.
  72. . Retrieved 16 April 2023.
  73. . Retrieved 2014-06-11.
  74. . Retrieved 2014-06-11.
  75. . Retrieved 16 April 2023.
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References

External links