Silurian

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Silurian
443.8 ± 1.5 – 419.2 ± 3.2 Ma
Silurian plate tectonics.png
Plate tectonics of Earth during the early Silurian
Chronology
Etymology
Name formalityFormal
Synonym(s)Gotlandian
Usage information
Celestial body Earth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unit Period
Stratigraphic unit System
First proposed by Roderick Murchison, 1835
Time span formalityFormal
Lower boundary definition FAD of the Graptolite Akidograptus ascensus
Lower boundary GSSP Dob's Linn, Moffat, UK
55°26′24″N3°16′12″W / 55.4400°N 3.2700°W / 55.4400; -3.2700
GSSP ratified1984 [4] [5]
Upper boundary definitionFAD of the Graptolite Monograptus uniformis
Upper boundary GSSP Klonk, Czech Republic
49°51′18″N13°47′31″E / 49.8550°N 13.7920°E / 49.8550; 13.7920
GSSP ratified1972 [6]
Atmospheric and climatic data
Sea level above present dayAround 180m, with short-term negative excursions [7]

The Silurian ( /sɪˈljʊəriən,s-/ sih-LYOOR-ee-ən, sy-) [8] [9] [10] is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. [11] The Silurian is the shortest period of the Paleozoic Era. 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 a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

Contents

One important event in this period was the initial establishment of terrestrial life: vascular plants emerged from more primitive land plants, dikaryan fungi started expanding and diversifying along with glomeromycotan fungi, and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized.

A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish.

History of study

The Silurian system was first identified by British 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. [12] 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. [13] 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 resolved the conflict by defining a new Ordovician system including the contested beds. [14] An alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland. [15]

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. [16] His interpretation was questioned in 1854 by Edward Forbes, [17] 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 Llandovery in Carmarthenshire, Wales
Aeronian 440.8
Telychian 438.5
Wenlock Sheinwoodian 433.4 Wenlock Edge, Shropshire, EnglandDuring the Wenlock, the oldest-known tracheophytes of the genus Cooksonia , appear. The complexity of slightly later Gondwana plants like Baragwanathia , which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or even Ordovician.[ citation needed ] The first terrestrial animals also appear in the Wenlock, represented by air-breathing millipedes from Scotland. [18]
Homerian 430.5
Ludlow Gorstian 427.4 Ludlow, Shropshire, England
Ludfordian 425.6 Ludford, Shropshire
Přídolí 423.0Named 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. [19]

Paleogeography

Ordovician-Silurian boundary on Hovedoya, Norway, showing brownish late Ordovician mudstone and later dark deep-water Silurian shale. The layers have been overturned by the Caledonian orogeny. Ordovicium-Silurian.jpg
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. [20] 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. [20]

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 glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. The continents of Avalonia, Baltica, and Laurentia 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 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 Late silurian sea bed arp.jpg
Fossils of the late Silurian sea bed

Climate and sea level

The Silurian Period likely 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. 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. [20]

During this period, the Earth entered a long, warm greenhouse phase, supported by high CO2 levels of 4500 ppm, and warm shallow seas covered much of the equatorial land masses. [21] Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. [22] 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. [23]

Perturbations

The climate and carbon cycle appear to be rather unsettled during the Silurian, which had a higher frequency of isotopic excursions (indicative of climate fluctuations) than any other period. [20] The Ireviken event, Mulde event and Lau event each represent isotopic excursions following a minor mass extinction [24] and associated with rapid sea-level change. Each one leaves a similar signature in the geological record, both geochemically and biologically; pelagic (free-swimming) organisms were particularly hard hit, as were brachiopods, corals and trilobites, and extinctions rarely occur in a rapid series of fast bursts. [20] [25] The climate fluctuations are best explained by a sequence of glaciations, but the lack of tillites in the middle to late Silurian make this explanation problematic. [26]

Flora and fauna

The Silurian was the first period to see megafossils of extensive terrestrial biota, in the form of moss-like miniature forests along lakes and streams. However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian. [20]

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. [27] 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, [28] Canada, [29] and China. [30] Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils [31] of Early Silurian (Llandovery) age. [32] The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae. [31]

The first bony fish, the Osteichthyes, appeared, represented by the Acanthodians covered with bony scales. Fish reached considerable diversity and developed movable jaws, adapted from the supports of the front two or three gill arches. A diverse fauna of eurypterids (sea scorpions)—some of them several meters in length—prowled the shallow Silurian seas of North America; many of their fossils have been found in New York state. Leeches also made their appearance during the Silurian Period. Brachiopods, bryozoa, molluscs, hederelloids, tentaculitoids, crinoids and trilobites were abundant and diverse.[ citation needed ] Endobiotic symbionts were common in the corals and stromatoporoids. [33] [34]

Reef abundance was patchy; sometimes, fossils are frequent, but at other points, are virtually absent from the rock record. [20]

The earliest-known animals fully adapted to terrestrial conditions appear during the Mid-Silurian, including the millipede Pneumodesmus . [18] Some evidence also suggests the presence of predatory trigonotarbid arachnoids and myriapods in Late Silurian facies. [35] Predatory invertebrates would indicate that simple food webs were in place that included non-predatory prey animals. Extrapolating back from Early Devonian biota, Andrew Jeram et al. in 1990 [36] suggested a food web based on as-yet-undiscovered detritivores and grazers on micro-organisms. [37]

Notes

  1. Jeppsson, L.; Calner, M. (2007). "The Silurian Mulde Event and a scenario for secundo—secundo events". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 93 (02): 135–154. doi:10.1017/S0263593300000377.
  2. Munnecke, A.; Samtleben, C.; Bickert, T. (2003). "The Ireviken Event in the lower Silurian of Gotland, Sweden-relation to similar Palaeozoic and Proterozoic events". Palaeogeography, Palaeoclimatology, Palaeoecology. 195 (1): 99–124. doi:10.1016/S0031-0182(03)00304-3.
  3. "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  4. Lucas, Sepncer (6 November 2018). "The GSSP Method of Chronostratigraphy: A Critical Review". Frontiers in Earth Science. 6: 191. Bibcode:2018FrEaS...6..191L. doi: 10.3389/feart.2018.00191 .
  5. Holland, C. (June 1985). "Series and Stages of the Silurian System" (PDF). Episodes. 8 (2): 101–103. doi: 10.18814/epiiugs/1985/v8i2/005 . 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. Haq, B. U.; Schutter, SR (2008). "A Chronology of Paleozoic Sea-Level Changes". Science. 322 (5898): 64–68. Bibcode:2008Sci...322...64H. doi:10.1126/science.1161648. PMID   18832639. S2CID   206514545.
  8. Wells, John (3 April 2008). Longman Pronunciation Dictionary (3rd ed.). Pearson Longman. ISBN   978-1-4058-8118-0.
  9. "Silurian". Dictionary.com Unabridged (Online). n.d.
  10. "Silurian". CollinsDictionary.com . HarperCollins.
  11. "International Chronostratigraphic Chart v.2015/01" (PDF). International Commission on Stratigraphy. January 2015.
  12. See:
  13. 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.
  14. Lapworth, Charles (1879). "On the tripartite classification of the Lower Palaeozoic rocks". Geological Magazine. 2nd series. 6 (1): 1–15. Bibcode:1879GeoM....6....1L. doi:10.1017/s0016756800156560. 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."
  15. The Gotlandian system was proposed in 1893 by the French geologist 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].)
  16. Barrande, Joachim (1852). Systême silurien du centre de la Bohême (in French). Paris, France and Prague, (Czech Republic): (Self-published). pp. ix–x.
  17. Forbes, Edward (1854). "Anniversary Address of the President". Quarterly Journal of the Geological Society of London. 10: xxii–lxxxi. See p. xxxiv.
  18. 1 2 Paul Selden & Helen Read (2008). "The oldest land animals: Silurian millipedes from Scotland" (PDF). Bulletin of the British Myriapod & Isopod Group. 23: 36–37.
  19. Manda, Štěpán; Frýda, Jiří (2010). "Silurian-Devonian boundary events and their influence on cephalopod evolution: evolutionary significance of cephalopod egg size during mass extinctions". Bulletin of Geosciences. 85 (3): 513–40. doi: 10.3140/bull.geosci.1174 .
  20. 1 2 3 4 5 6 7 Munnecke, Axel; Calner, Mikael; Harper, David A.T.; Servais, Thomas (2010). "Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology. 296 (3–4): 389–413. Bibcode:2010PPP...296..389M. doi:10.1016/j.palaeo.2010.08.001.
  21. Chaloner, William G. (2003). "The role of carbon dioxide in plant evolution". Evolution on Planet Earth: 65–83. doi:10.1016/B978-012598655-7/50032-X. ISBN   9780125986557.
  22. Gambacorta, G.; Menichetti, E.; Trincianti, E.; Torricelli, S. (March 2019). "The Silurian climatic transition recorded in the epicontinental Baltica Sea". Palaeogeography, Palaeoclimatology, Palaeoecology. 517: 16–29. Bibcode:2019PPP...517...16G. doi:10.1016/j.palaeo.2018.12.016. S2CID   135118794.
  23. Nealon, T.; Williams, D. Michael (30 April 2007). "Storm-influenced shelf deposits from the silurian of Western Ireland: A reinterpretation of deep basin sediments". Geological Journal. 23 (4): 311–320. doi:10.1002/gj.3350230403.
  24. Samtleben, C.; Munnecke, A.; Bickert, T. (2000). "Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment". Facies. 43: 1–38. doi:10.1007/BF02536983. S2CID   130640332.
  25. Trotter, Julie A.; Williams, Ian S.; Barnes, Christopher R.; Männik, Peep; Simpson, Andrew (February 2016). "New conodont δ18O records of Silurian climate change: Implications for environmental and biological events". Palaeogeography, Palaeoclimatology, Palaeoecology. 443: 34–48. Bibcode:2016PPP...443...34T. doi:10.1016/j.palaeo.2015.11.011.
  26. Calner, Mikael (2008). "Silurian global events – at the tipping point of climate change". Mass Extinction: 21–57. doi:10.1007/978-3-540-75916-4_4. ISBN   978-3-540-75915-7.
  27. Rittner, Don (2009). Encyclopedia of Biology. Infobase Publishing. p. 338. ISBN   9781438109992.
  28. Lang, W.H.; Cookson, I.C. (1935). "On a flora, including vascular land plants, associated with Monograptus, in rocks of Silurian age, from Victoria, Australia". Philosophical Transactions of the Royal Society of London B. 224 (517): 421–449. Bibcode:1935RSPTB.224..421L. doi: 10.1098/rstb.1935.0004 .
  29. Hueber, F.M. (1983). "A new species of Baragwanathia from the Sextant Formation (Emsian) Northern Ontario, Canada". Botanical Journal of the Linnean Society. 86 (1–2): 57–79. doi:10.1111/j.1095-8339.1983.tb00717.x.
  30. Bora, Lily (2010). Principles of Paleobotany. Mittal Publications. pp. 36–37.
  31. 1 2 Niklas, Karl J. (1976). "Chemical Examinations of Some Non-Vascular Paleozoic Plants". Brittonia . 28 (1): 113–137. doi:10.2307/2805564. JSTOR   2805564. S2CID   21794174.
  32. Edwards, D. & Wellman, C. (2001), "Embryophytes on Land: The Ordovician to Lochkovian (Lower Devonian) Record", in Gensel, P. & Edwards, D. (eds.), Plants Invade the Land : Evolutionary and Environmental Perspectives, New York: Columbia University Press, pp. 3–28, ISBN   978-0-231-11161-4 , p. 4
  33. Vinn, O.; wilson, M.A.; Mõtus, M.-A. (2014). "Symbiotic endobiont biofacies in the Silurian of Baltica". Palaeogeography, Palaeoclimatology, Palaeoecology. 404: 24–29. Bibcode:2014PPP...404...24V. doi:10.1016/j.palaeo.2014.03.041 . Retrieved 2014-06-11.
  34. Vinn, O.; Mõtus, M.-A. (2008). "The earliest endosymbiotic mineralized tubeworms from the Silurian of Podolia, Ukraine". Journal of Paleontology. 82 (2): 409–414. doi:10.1666/07-056.1. S2CID   131651974 . Retrieved 2014-06-11.
  35. Garwood, Russell J.; Edgecombe, Gregory D. (September 2011). "Early Terrestrial Animals, Evolution, and Uncertainty". Evolution: Education and Outreach. 4 (3): 489–501. doi: 10.1007/s12052-011-0357-y .
  36. Jeram, Andrew J.; Selden, Paul A.; Edwards, Dianne (1990). "Land Animals in the Silurian: Arachnids and Myriapods from Shropshire, England". Science. 250 (4981): 658–61. Bibcode:1990Sci...250..658J. doi:10.1126/science.250.4981.658. PMID   17810866.
  37. DiMichele, William A; Hook, Robert W (1992). "The Silurian". In Behrensmeyer, Anna K. (ed.). Terrestrial Ecosystems Through Time: Evolutionary Paleoecology of Terrestrial Plants and Animals. pp. 207–10. ISBN   978-0-226-04155-1.

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<i>Chaetosalpinx</i> Trace fossil

Chaetosalpinx is an ichnogenus of bioclaustrations. Chaetosalpinx includes straight to sinuous cavities that are parallel to the host's axis of growth. The cavity is circular to oval in cross-section and it lacks a wall lining or floor-like tabulae. They are common in tabulate and rugose corals from Late Ordovician to Devonian of Europe and North America. They may have been parasites.

The Late Ordovician glaciation, also known as the Hirnantian glaciation or end-Ordovician glaciation, is the first part of the Andean-Saharan glaciation. It was centered on the Sahara region in late Ordovician, about 440–460 Ma. The major glaciation during this period is widely considered to be the leading cause of the Ordovician-Silurian extinction event. Evidence of this glaciation can be seen in places such as Morocco, South Africa, Libya, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5 °C cooler than present day; this would have been a major factor that aided in the glaciation process.

Olev Vinn is Estonian paleobiologist and paleontologist.

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