The Silurian 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. 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 several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out.
A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life also began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes, streams, and coastlines, and terrestrial arthropods are also first found on land during the Silurian. However, terrestrial life would not greatly diversify and affect the landscape until the Devonian.
443.8–419.2 million years ago
|Mean atmospheric O
2 content over period duration
|c. 14 vol %|
(70 % of modern level)
|Mean atmospheric CO
2 content over period duration
|c. 4500 ppm|
(16 times pre-industrial level)
|Mean surface temperature over period duration||c. 17 °C|
(3 °C above modern level)
|Sea level (above present day)||Around 180 m, with short-term negative excursions|
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. 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. 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. 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 divided the 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.
The Llandovery Epoch lasted from to mya, and is subdivided into three stages: the Rhuddanian, lasting until , the Aeronian, lasting to , and the Telychian. The epoch is named for the town of Llandovery in Carmarthenshire, Wales.
The Wenlock, which lasted from to mya, is subdivided into the Sheinwoodian (to ) and Homerian ages. It is named after Wenlock Edge in 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. The first terrestrial animals also appear in the Wenlock, represented by air-breathing millipedes from Scotland.
In North America a different suite of regional stages is sometimes used:
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. 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.
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.
The Silurian period 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 can be identified, and the highest Silurian sea level was probably around 140 m higher than the lowest level reached.
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. Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. 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. Later in the Silurian, the climate cooled slightly, but closer to the Silurian-Devonian boundary, the climate became warmer.
The climate and carbon cycle appears to be rather unsettled during the Silurian, which has a higher concentration of isotopic excursions than any other period. The Ireviken event, Mulde event and Lau event each represent isotopic excursions following a minor mass extinction and associated with rapid sea-level change, in addition to the larger extinction at the end of the Silurian. 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.
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.
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. 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 cm. 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, Canada and China. Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils of early Silurian (Llandovery) age. The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae.
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. Endobiotic symbionts were common in the corals and stromatoporoids.
Reef abundance was patchy; sometimes fossils are frequent but at other points are virtually absent from the rock record.
The earliest-known animals fully adapted to terrestrial conditions appear during the Mid-Silurian, including the millipede Pneumodesmus. Some evidence also suggests the presence of predatory trigonotarbid arachnoids and myriapods in Late Silurian facies. 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 suggested a food web based on as-yet-undiscovered detritivores and grazers on micro-organisms.
Actinopterygii (), or the ray-finned fishes, constitute a class or subclass of the bony fishes.The ray-finned fishes are so called because their fins are webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii (lobe-finned fish). These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).
Numerically, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish. They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the massive ocean sunfish, at 2,300 kg (5,070 lb), and the long-bodied oarfish, at 11 m (36 ft).Aeronian
In the geologic timescale, the Aeronian is the age of the Llandovery epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon that began 440.8 ± 1.2 Ma and ended 438.5 ± 1.1 Ma (million years ago). The Aeronian age succeeds the Rhuddanian age and precedes the Telychian age, all in the same epoch.Caledonian orogeny
The Caledonian orogeny was a mountain-building era recorded in the northern parts of Ireland and Britain, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the continents and terranes of Laurentia, Baltica and Avalonia collided.
The Caledonian orogeny is named for Caledonia, the Latin name for Scotland. The name was first used in 1885 by Austrian geologist Eduard Suess for an episode of mountain building in northern Europe that predated the Devonian period. Geologists like Émile Haug and Hans Stille saw the Caledonian orogeny as one of several episodic phases of mountain building that had occurred during Earth's history. Current understanding has it that the Caledonian orogeny encompasses a number of tectonic phases that can laterally be diachronous. The name "Caledonian" can therefore not be used for an absolute period of geological time, it applies only to a series of tectonically related events.Euteleostomi
Euteleostomi is a successful clade that includes more than 90% of the living species of vertebrates. Euteleostomes are also known as "bony vertebrates". Both its major subgroups are successful today: Actinopterygii includes the majority of extant fish species, and Sarcopterygii includes the tetrapods.
"Osteichthyes" in the paleontological sense (i.e., "bony vertebrates"), is synonymous with Euteleostomi. However, in ichthyology and Linnaean taxonomy Osteichthyes, literally "bony fish," refers to the paraphyletic group that differs by excluding tetrapods. The name Euteleostomi, coined as a monophyletic alternative that unambiguously includes the living tetrapods, is more widely used in bioinformatics and related fields. The term Euteleostomi comes from Eu-teleostomi, where "Eu-" comes from the Greek εὖ meaning well or good, so the clade can be defined as the living teleostomes.
Euteleostomes originally all had endochondral bone, fins with lepidotrichs (fin rays), jaws lined by maxillary, premaxillary, and dentary bones composed of dermal bone, and lungs. Many of these characters have since been lost by descendant groups, however, such as lepidotrichs lost in tetrapods, and bone lost among the chondrostean fishes. Lungs have been retained in dipnoi (lungfish), and many tetrapods (birds, mammals, reptiles, and some amphibians). In many ray-finned fishes lungs have evolved into swim bladders for regulating buoyancy, while in others they continue to be used as respiratory gas bladders.Gorstian
In the geologic timescale, the Gorstian is the age of the Ludlow epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon that is comprehended between 422.9 ± 2.5 Ma and 421.3 ± 2.6 Ma (million years ago), approximately. The Gorstian age succeeds the Homerian age and precedes the Ludfordian age.Llandovery epoch
In the geological timescale, the Llandovery epoch (from 443.7 ± 1.5 million years ago to 428.2 ± 2.3 million years ago) occurred during the Silurian period. The epoch follows the massive Ordovician-Silurian extinction events; the second largest extinction event after the Permian-Triassic extinction event million of years later.
The epoch was named after Llandovery in Wales.Ludlow epoch
In the geological timescale, the Ludlow epoch (from 422.9 ± 2.5 million years ago to 418.7 ± 2.7 million years ago) occurred during the Silurian period, after the end of the Homerian age. It is named for the town of Ludlow in Shropshire, England.
The Ludlow epoch is subdivided into two stages: Gorstian and Ludfordian.Lycopodiophyta
The Division Lycopodiophyta (sometimes called lycophyta or lycopods) is a tracheophyte subgroup of the Kingdom Plantae. It is one of the oldest lineages of extant (living) vascular plants and contains extinct plants like Baragwanathia that have been dated from the Silurian (ca. 425 million years ago). These species reproduce by shedding spores and have macroscopic alternation of generations, although some are homosporous while others are heterosporous. Most members of Lycopodiophyta bear a protostele, and the sporophyte generation is dominant. They differ from all other vascular plants in having microphylls, leaves that have only a single vascular trace (vein) rather than the much more complex megaphylls found in ferns and seed plants.Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains over 16,000 species, most of which are terrestrial. Although their name suggests they have myriad (10,000) legs, myriapods range from having up to 750 legs (the millipede Illacme plenipes) to having fewer than ten legs.
The fossil record of myriapods reaches back into the late Silurian, although molecular evidence suggests a diversification in the Cambrian Period, and Cambrian fossils exist which resemble myriapods. The oldest unequivocal myriapod fossil is of the millipede Pneumodesmus newmani, from the late Silurian (428 million years ago). P. newmani is also important as the earliest known terrestrial animal. The phylogenetic classification of myriapods is still debated.
The scientific study of myriapods is myriapodology, and those who study myriapods are myriapodologists.Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also extends northwards into Greenland and Svalbard. In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology. For convenience the short version of the term, ORS is often used in literature on the subject. The term was coined to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.
Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today.Ordovician–Silurian extinction events
The Ordovician–Silurian extinction events, when combined, are the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. This event greatly affected marine communities, which caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. The Ordovician–Silurian extinction occurred during the Hirnantian stage of the Ordovician Period and the subsequent Rhuddanian stage of the Silurian Period. The last event is dated in the interval of 455–430 Ma ago, i.e., lasting from the Middle Ordovician to Early Silurian, thus including the extinction period. This event was the first of the big five Phanerozoic events and was the first to significantly affect animal-based communities.Almost all major taxonomic groups were affected during this extinction event. Extinction was global during this period, eliminating 49-60% of marine genera and nearly 85% of marine species.Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. Before the late Ordovician cooling, temperatures were relatively warm and it is the suddenness of the climate changes and the elimination of habitats due to sea-level fall that are believed to have precipitated the extinctions. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling were likely driving agents for the Ordovician mass extinction.Paleozoic
The Paleozoic (or Palaeozoic) Era ( ; from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from 541 to 251.902 million years ago, and is subdivided into six geologic periods (from oldest to youngest): the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic comes after the Neoproterozoic Era of the Proterozoic Eon and is followed by the Mesozoic Era.
The Paleozoic was a time of dramatic geological, climatic, and evolutionary change. The Cambrian witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Arthropods, molluscs, fish, amphibians, synapsids and diapsids all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms. Great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated diapsids and synapsids were dominant and the first modern plants (conifers) appeared.
The Paleozoic Era ended with the largest extinction event in the history of Earth, the Permian–Triassic extinction event. The effects of this catastrophe were so devastating that it took life on land 30 million years into the Mesozoic Era to recover. Recovery of life in the sea may have been much faster.Pridoli epoch
In the geologic timescale, the Pridoli epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon is comprehended between 423 ± 1.5 and 419.2 ± 2.8 mya (million years ago), approximately. The Pridoli epoch succeeds the Ludfordian age and precedes the Lochkovian age of the Devonian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area.Rochester Shale
The Rochester Shale is a geologic formation in West Virginia. It preserves fossils dating back to the Silurian period.Sarcopterygii
The Sarcopterygii () or lobe-finned fish (from Greek σάρξ sarx, flesh, and πτέρυξ pteryx, fin)—sometimes considered synonymous with Crossopterygii ("fringe-finned fish", from Greek κροσσός krossos, fringe)—constitute a clade (traditionally a class or subclass) of the bony fish, though a strict cladistic view includes the terrestrial vertebrates.
The living sarcopterygians include two species of coelacanths and six species of lungfish.Silurian (Doctor Who)
The Silurians are a fictional race of reptile-like humanoids in the long-running British science fiction television series Doctor Who. The species first appeared in Doctor Who in the 1970 serial Doctor Who and the Silurians, and were created by Malcolm Hulke. The first Silurians introduced are depicted as prehistoric and scientifically advanced sentient humanoids who predate the dawn of man; in their backstory, the Silurians went into self-induced hibernation to survive what they predicted to be a large atmospheric upheaval caused by the Earth capturing the Moon. The Silurians introduced in the 1970 story are broad, three-eyed land-dwellers. The 1972 serial The Sea Devils, also by Hulke, introduced their amphibious cousins, the so-called "Sea Devils". Both Silurians and Sea Devils made an appearance in 1984's Warriors of the Deep. After Warriors of the Deep, the Silurians did not appear in the show again before its 1989 cancellation. Heavily redesigned Silurans were reintroduced to the series in 2010, following the show's 2005 revival, and have recurred frequently since then.
Commonly called Silurians, after their supposed origins in the Silurian period, the creatures have also been referred to by other names. In The Sea Devils, the Third Doctor (Jon Pertwee) claims that "properly speaking", the Silurians should have been called "Eocenes". The name Homo reptilia is first used to describe the creatures in the novelisation Doctor Who and the Cave-Monsters (1974), and is first used in the series proper in the episode "The Hungry Earth" (2010). In The Sea Devils, an amphibious Silurian is dubbed a "Sea Devil" by the human workman Clark (Declan Mulholland), while in Warriors of the Deep, the land-dwelling Silurians use the term "Sea Devil" to refer to their aquatic counterparts.Vascular plant
Vascular plants (from Latin vasculum: duct), also known as tracheophytes (from the equivalent Greek term trachea) and also as higher plants, form a large group of plants (c. 308,312 accepted known species) that are defined as those land plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue (the phloem) to conduct products of photosynthesis. Vascular plants include the clubmosses, horsetails, ferns, gymnosperms (including conifers) and angiosperms (flowering plants). Scientific names for the group include Tracheophyta, Tracheobionta and Equisetopsida sensu latoWenlock epoch
The Wenlock (sometimes referred to as the Wenlockian) is the second series of the Silurian. It is preceded by the Llandovery epoch and followed by the Ludlow Group. Radiometric dates constrain the Wenlockian between 433.4 and 427.4 million years ago.