Silurian

Template:Short description Template:Distinguish {{#invoke:other uses|otheruses}} Template:Infobox geologic timespan

The Silurian (Template:IPAc-en Template:Respell)<ref>Template:Cite book</ref><ref>Template:Cite Dictionary.com</ref><ref>Template:Cite Collins Dictionary</ref> is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at Template:Period start million years ago (Mya), to the beginning of the Devonian Period, Template:Period end Mya.<ref name="ICS2024-12">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Silurian is the third and shortest period of the Paleozoic Era, and the third of twelve periods of the Phanerozoic Eon. 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.

One important event in this period was the initial establishment of terrestrial life in what is known as the Silurian-Devonian Terrestrial Revolution: vascular plants emerged from more primitive land plants,<ref name="SilurianDevonianTerrestrialRevolution" /><ref name="SilurianDevonianTerrestrialRevolutionChina" /> dikaryan fungi started expanding and diversifying along with glomeromycotan fungi,<ref name="LutzoniEtAl2018">Template:Cite journal</ref> and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized.<ref name="Garwood">Template:Cite journal</ref>

Another significant evolutionary milestone during the Silurian was the diversification of jawed fish, which include placoderms, acanthodians (which gave rise to cartilaginous fish) and osteichthyan (bony fish, further divided into lobe-finned and ray-finned fishes),<ref name="Brazeau-Friedman-2015">Template:Cite journal</ref> although this corresponded to sharp decline of jawless fish such as conodonts and ostracoderms.

History of studyEdit

Template:Life timeline 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 a Latin name for Wales.<ref>See:

Template:Cite journal From p. 48: " … I venture to suggest, that as the great mass of rocks in question, trending from south-west to north-east, traverses the kingdom of our ancestors the Silures, the term "Silurian system" should be adopted … "
Template:Cite book</ref> Whilst the British rocks now identified as belonging to the Silurian System and the lands now thought to have been inhabited in antiquity by the Silures show little correlation ([[wiktionary:cf|Template:Abbr]]. Geologic map of Wales, Map of pre-Roman tribes of Wales), Murchison conjectured that their territory included Caer Caradoc and Wenlock Edge exposures - and that if it did not there were plenty of Silurian rocks elsewhere 'to sanction the name proposed'.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal 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."</ref> An alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland.<ref>The Gotlandian system was proposed in 1893 by the French geologist Albert Auguste Cochon de Lapparent (1839–1908): Template:Cite book 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].)</ref>

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.<ref>Template:Cite book</ref> His interpretation was questioned in 1854 by Edward Forbes,<ref>Template:Cite journal See p. xxxiv.</ref> 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.

SubdivisionsEdit

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 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.Template:Citation needed The first terrestrial animals also appear in the Wenlock, represented by air-breathing millipedes from Scotland.<ref name=Selden&Read>Template:Cite journal</ref>
Wenlock	Homerian	430.5	Homer, Shropshire, England
Ludlow	Gorstian	427.4	Gorsty village near Ludlow, Shropshire, England
Ludlow	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.<ref>Template:Cite journal</ref>

PaleogeographyEdit

File: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.<ref name=Munnecke2010/> 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.<ref name=Munnecke2010/>

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.

File:Late silurian sea bed arp.jpg

Fossils of the late Silurian sea bed, England

Climate and sea levelEdit

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,<ref name="Yan2022EarlySilurianClimateChange">Template:Cite journal</ref><ref name="GambacortaMenichettiTrinciantiTorricelli" /> evidenced by numerous major carbon and oxygen isotope excursions during this geologic period.<ref name="YoungEtAl2020PPP">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="TrotterEtAl2016PPP" /> 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 Template:Convert higher than the lowest level reached.<ref name=Munnecke2010/>

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.<ref name="Chaloner2003">Template:Cite journal</ref> Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian.<ref name="GambacortaMenichettiTrinciantiTorricelli">Template:Cite journal</ref> Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.<ref>Template:Cite journal</ref>

PerturbationsEdit

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.<ref name=Munnecke2010/> The Ireviken event, Mulde event, and Lau event each represent isotopic excursions following a minor mass extinction<ref>Template:Cite journal</ref> 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.<ref name=Munnecke2010/><ref name="TrotterEtAl2016PPP">Template:Cite journal</ref> 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.<ref name="Calner2008">Template:Cite book</ref>

Flora and faunaEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} 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.<ref>Template:Cite journal</ref>

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 and networks of large, mycorrhizal nematophytes, heralding the beginning of the Silurian-Devonian Terrestrial Revolution.<ref name="SilurianDevonianTerrestrialRevolution">Template:Cite journal</ref><ref name="SilurianDevonianTerrestrialRevolutionChina">Template:Cite journal</ref><ref>Template:Cite journal</ref> However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian.<ref name=Munnecke2010>Template:Cite journal</ref>

File:Diorama of a Silurian seafloor - cephalopod, trilobites, fish, crinoids, gastropods, corals, algae, starfish, brachiopods, bryozoans (44982225194).jpg

Diorama of a Silurian seafloor

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.<ref name="DonRittnerEncyclopaedia">Template:Cite book</ref> 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)Template:Update inline and has branching stems and needle-like leaves of Template:Convert. 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,<ref name="RickardsBaragwanathia2000">Template:Cite journal</ref><ref name=Lang>Template:Cite journal</ref> Canada,<ref name=Hueber1983>Template:Cite journal</ref> and China.<ref>Template:Cite book</ref> Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils<ref name='Niklas1976'>Template:Cite journal</ref> of Early Silurian (Llandovery) age.<ref>Template:Citation, p. 4</ref> The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae.<ref name=Niklas1976/>

Fossils that are considered as terrestrial animals are also known from the Silurian. The definitive oldest record of millipede ever known is Kampecaris obanensis and Archidesmus sp. from the late Silurian (425 million years ago) of Kerrera.<ref name="Brookfield-2021">Template:Cite journal</ref> There are also other millipedes, centipedes, and trigonotarbid arachnoids known from Ludlow (420 million years ago).<ref name="Brookfield-2021" /><ref>Template:Cite journal</ref><ref>Template:Cite book</ref> 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<ref>Template:Cite journal</ref> suggested a food web based on as-yet-undiscovered detritivores and grazers on micro-organisms.<ref name="DiMicheleBookSilurian">Template:Cite book</ref> Millipedes from Cowie Formation such as Cowiedesmus and Pneumodesmus were considered as the oldest millipede from the middle Silurian at 428–430 million years ago,<ref name="Selden&Read" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> although the age of this formation is later reinterpreted to be from the early Devonian instead by some researchers.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Regardless, Pneumodesmus is still an important fossil as the oldest definitive evidence of spiracles to breathe in the air.<ref name="Brookfield-2021" />

File:Life restoration of Sparalepis tingi.tiff

Life restoration of sarcopterygian Sparalepis tingi and other fauna from the Silurian of Yunnan

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 a few meters in length—prowled the shallow Silurian seas and lakes of North America; many of their fossils have been found in New York state. Brachiopods were abundant and diverse, with the taxonomic composition, ecology, and biodiversity of Silurian brachiopods mirroring Ordovician ones.<ref>Template:Cite journal</ref> Brachiopods that survived the LOME developed novel adaptations for environmental stress,<ref>Template:Cite journal</ref> and they tended to be endemic to a single palaeoplate in the mass extinction's aftermath, but expanded their range afterwards.<ref>Template:Cite journal</ref> The most abundant brachiopods were atrypids and pentamerides;<ref name="HuangJinRong2018" /> atrypids were the first to recover and rediversify in the Rhuddanian after LOME,<ref>Template:Cite journal</ref> while pentameride recovery was delayed until the Aeronian.<ref name="HuangJinRong2018">Template:Cite journal</ref> Bryozoans exhibited significant degrees of endemism to a particular shelf.<ref>Template:Cite journal</ref> They also developed symbiotic relationships with cnidarians<ref>Template:Cite journal</ref> and stromatolites.<ref>Template:Cite journal</ref> Many bivalve fossils have also been found in Silurian deposits,<ref>Template:Cite journal</ref> and the first deep-boring bivalves are known from this period.<ref>Template:Cite journal</ref> Chitons saw a peak in diversity during the middle of the Silurian.<ref>Template:Cite journal</ref> Hederelloids enjoyed significant success in the Silurian, with some developing symbioses with the colonial rugose coral Entelophyllum.<ref name="HederelloidRugosanSymbiosis">Template:Cite journal</ref> The Silurian was a heyday for tentaculitoids,<ref>Template:Cite journal</ref> which experienced an evolutionary radiation focused mainly in Baltoscandia,<ref>Template:Cite journal</ref> along with an expansion of their geographic range in the Llandovery and Wenlock.<ref>Template:Cite journal</ref> Trilobites started to recover in the Rhuddanian,<ref>Template:Cite journal</ref> and they continued to enjoy success in the Silurian as they had in the Ordovician despite their reduction in clade diversity as a result of LOME.<ref>Template:Cite journal</ref> The Early Silurian was a chaotic time of turnover for crinoids as they rediversified after LOME.<ref>Template:Cite journal</ref> Members of Flexibilia, which were minimally impacted by LOME, took on an increasing ecological prominence in Silurian seas.<ref>Template:Cite journal</ref> Monobathrid camerates, like flexibles, diversified in the Llandovery, whereas cyathocrinids and dendrocrinids diversified later in the Silurian.<ref>Template:Cite journal</ref> Scyphocrinoid loboliths suddenly appeared in the terminal Silurian, shortly before the Silurian-Devonian boundary, and disappeared as abruptly as they appeared very shortly after their first appearance.<ref name="ZongGong-ZongAndGong">Template:Cite journal</ref> Endobiotic symbionts were common in the corals and stromatoporoids.<ref name='VinnWilsonMotus2014'>Template:Cite journal</ref><ref name='VinnMotus2008'>Template:Cite journal</ref> Rugose corals especially were colonised and encrusted by a diverse range of epibionts,<ref>Template:Cite journal</ref> including certain hederelloids as aforementioned.<ref name="HederelloidRugosanSymbiosis" /> Photosymbiotic scleractinians made their first appearance during the Middle Silurian.<ref>Template:Cite journal</ref> Reef abundance was patchy; sometimes, fossils are frequent, but at other points, are virtually absent from the rock record.<ref name=Munnecke2010/>

Cooksonia sp. - MUSE.jpg

Cooksonia, the earliest vascular plant, middle Silurian
Wrens Nest Fossils 2.jpg

Silurian sea bed fossils collected from Wren's Nest Nature Reserve, Dudley UK
Kaugatuma Bedding Plane Pridoli Estonia.jpg

Crinoid fragments in a Silurian (Pridoli) limestone (Saaremaa, Estonia)
Wrens Nest Fossils 3.jpg

Silurian sea bed fossils collected from Wren's Nest Nature Reserve, Dudley UK
Eurypterus Paleoart.jpg

Eurypterus, a common Upper Silurian eurypterid
20201227 Pterygotus anglicus.png

Pterygotus was a giant eurypterid that had a nearly cosmopolitan distribution (reconstruction shown here is Devonian species P. anglicus)
Calymene celebra Raymond, 1916.jpg

Trilobites were still diverse and common in the Silurian. Fossils of Calymene celebra are extremely abundant in parts of central US.
HalysitesSilurian.jpg

Halysites was a Tabulate coral, an extinct group that lived through the Paleozoic
20211029 Parioscorpio venator.png

Parioscorpio was an enigmatic arthropod from the Silurian of Wisconsin
Dalmanites limulurus trilobite silurian.jpg

A Dalmanites limulurus specimen from Silurian strata of New York
Geodized pentamerid brachiopods (Silurian; Swayzee, Indiana, USA) 1.jpg

A rock containing several geodized pentamerid brachiopods from strata in Indiana
Sphooceras-truncatum.jpg

Sphooceras was a Nautiloid cephalopod found in Silurian strata of the Czech Republic
Jamoytius kerwoodi.jpg

Jamoytius was an enigmatic vertebrate that is possibly related to Anaspid fish
Poraspis.jpg

Poraspis, a genus of armored jawless fish from the Late Silurian of Canada, Norway, and the U.S.
Tujiaaspis.jpg

Tujiaaspis is a galeaspid agnathan from the early Silurian (Telychian) of China, showing origin of paired fins
Qianodus holotype.jpg

Qianodus<ref>Template:Cite journal</ref> is a tooth-based chondrichthyan genus from the early Silurian (Aeronian) of China
Fanjingshania.jpg

Fanjingshania, climatiid spiny shark from the lower Silurian (Aeronian) described from disarticulated dermoskeletal elements
Shenacanthus vermiformis.jpg

Shenacanthus<ref name=":0">Template:Cite journal</ref> is jawed stem-chondrichthyan genus from the early Silurian (Telychian) of China
Xiushanosteus.jpg

Xiushanosteus<ref name=":0" /> is the oldest known placoderm from the early Silurian (Telychian) of China
Reconstruction of Entelognathus primordialis in lateral view.png

Entelognathus primordialis was a Placoderm fish from the late Silurian

NotesEdit

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ReferencesEdit

Emiliani, Cesare. (1992). Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment. Cambridge University Press. (Paperback Edition Template:ISBN)
Mikulic, DG, DEG Briggs, and J Kluessendorf. 1985. A new exceptionally preserved biota from the Lower Silurian of Wisconsin, USA. Philosophical Transactions of the Royal Society of London, 311B:75-86.
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