Silurian Period
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What is the Silurian Period?
The Silurian Period, the third period of the Paleozoic Era, began 443.8 million years ago and ended 419.2 million years ago. It followed the Ordovician Period and preceded the Devonian Period.
During the Silurian, sea levels rose due to the melting of glaciers from the Late Ordovician ice age, flooding large expanses of continents with shallow seas and creating common mound-type coral reefs. Fishes became widespread, and vascular plants began to colonize coastal lowlands, though continental interiors remained barren.
Scottish geologist Roderick I. Murchison named the Silurian Period in 1835 after a sequence of rocks in Wales, honoring the Silures, who resisted Roman conquest. Conodonts are important index fossils for correlating Silurian strata, with global correlation based on 12 1/2 conodont biozones.
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When did the Silurian Period occur?
The Silurian Period, the third period of the Paleozoic Era, began 443.8 million years ago and ended 419.2 million years ago. Named in 1835 by Scottish geologist Roderick I. Murchison, the Silurian Period takes its name from the Silures, an ancient people of Wales.
During this period, continental elevations were generally lower, and global sea levels were higher than today. As glaciers from the Late Ordovician ice age melted, sea levels rose dramatically, changing climatic conditions and allowing many faunal groups to recover from prior extinctions. Shallow seas flooded large expanses of several continents, and coral reefs were common. Fishes became widespread, and vascular plants began to colonize coastal lowlands.
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What major events happened during the Silurian Period?
The Silurian Period, the third period of the Paleozoic Era, spanned from 443.8 million to 419.2 million years ago. During this period, continental elevations were generally lower, and global sea levels were higher. As glaciers from the Late Ordovician ice age melted, sea levels rose dramatically, changing climatic conditions and allowing faunal groups to recover from extinctions.
Vascular plants began to colonize coastal lowlands, but continental interiors remained barren. Fishes were widespread, and reef mounds on the seafloor contained brachiopods, gastropods, crinoids, and trilobites. A variety of jawless fishes appeared, as did fishes with primitive jaws. Smaller sea level fluctuations occurred globally throughout the Silurian, not strongly affecting shelly bottom-dwelling invertebrates.
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How do scientists divide the Silurian Period into different stages?
The Silurian Period, lasting from 443.8 million to 419.2 million years ago, is divided into four series, which are further divided into stages. These divisions are based on the Silurian System, the corresponding rock layers from this period.
- Llandovery Series Includes the Rhuddanian, Aeronian, and Telychian stages.
- Wenlock Series Includes the Sheinwoodian and Homerian stages.
- Ludlow Series Includes the Gorstian and Ludfordian stages.
- Pridoli Series This series has not been divided into stages.
These series are defined using basal stratotypes, which are assemblages of fossils whose first appearance defines the beginning of a time interval. Brachiopods are effective index fossils for correlating Silurian strata. Absolute dates are calculated using radioisotopes from igneous rock samples, with adjustments made for metamorphism.
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What types of fossils help geologists study the Silurian Period?
During the Silurian Period, geologists rely on various types of fossils to study and correlate strata, including graptolites, brachiopods, and conodonts. Graptolites, small colonial planktonic animals, are abundant in dark Silurian shales and serve as excellent index fossils due to their rapid evolution and broad distribution. Brachiopods, particularly abundant and diverse during the Silurian, are effective index fossils in relatively shallow platform carbonates and clastic wedges. Conodonts, phosphatic microfossils, are common in Silurian rocks and are recovered from limestone or lime-cemented sediments. These fossils aid in correlating Silurian strata and understanding the ecological zonation of marine benthic invertebrates.
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Why is the Silurian Period important in Earth's history?
The Silurian Period, a segment of geologic time within the Paleozoic Era, spans from 443.8 million to 419.2 million years ago. It is notable for the recovery of many faunal groups from the Late Ordovician extinctions, spurred by rising sea levels as glaciers melted and climatic conditions changed.
During this period, continental elevations were generally lower, and global sea levels were higher than today. Large continental expanses became shallow seas, fostering the proliferation of mound-type coral reefs. Fishes became widespread, and vascular plants began to colonize coastal lowlands, though continental interiors remained largely barren. The distribution of marine invertebrate communities during the Silurian Period was influenced by ecological zonation, with different communities adapting to varying conditions of sunlight, wave energy, and water depth.
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Silurian Period, in geologic time, the third period of the Paleozoic Era. It began 443.8 million years ago and ended 419.2 million years ago, extending from the close of the Ordovician Period to the beginning of the Devonian Period.
During the Silurian, continental elevations were generally much lower than in the present day, and global sea level was much higher. Sea level rose dramatically as the extensive glaciers from the Late Ordovician ice age melted. This rising prompted changes in climatic conditions that allowed many faunal groups to recover from the extinctions of Late Ordovician times. Large expanses of several continents became flooded with shallow seas, and mound-type coral reefs were very common. Fishes were widespread. Vascular plants began to colonize coastal lowlands during the Silurian Period, whereas continental interiors remained essentially barren of life.
The name of this period is derived from work done by Scottish geologist Roderick I. Murchison, who in 1835 named a sequence of rocks in Wales and its borderland with England in honour of a native people called the Silures. The Silures, under the leadership of Caratacus, resisted Roman conquest for 30 years until 78 ce, when they were finally overcome.
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The Silurian environment
Paleogeography
During most of the Silurian Period, the vast Panthalassic Ocean covered the northern polar regions, the supercontinent of Gondwana stretched over the southern polar region, and a ring of at least six continents spanned the Equator and middle latitudes. The approximate orientations and locations of Silurian continents can be reconstructed using a combination of paleomagnetic, paleoclimatic, and biogeographic data. The Earth’s magnetic field leaves its signature on volcanic rocks and certain sedimentary rocks rich in such iron-bearing minerals as magnetite. As rocks capable of being magnetized are cooled or otherwise lithified, their component crystals (grains) are lined up with the Earth’s magnetic field. Unless the rocks are remelted by the heat from the planet’s interior or reworked by erosion, they retain this signature regardless of whether they change position or not. The Earth’s zonal climate also has an effect on global patterns of sedimentation.
The most unusual features of the Silurian that distinguish it from the present-day physical environment relate to conditions of low continental elevations combined with a much higher global stand in sea level. Extensive continental regions were flooded by shallow seas ranging in water depth from a few to little more than 100 metres (330 feet). Where these seas occupied a tropical to subtropical climatic zone, coral mound reefs with associated carbonate sediments were very common. Strata forming in arid regions differ from those formed in inundated areas or other regions with high annual rainfall. The deposition of evaporites (salts) was periodically set in motion as a result of reduced ocean circulation occurring in geographically restricted places such as shallow embayments.
The strong faunal endemism (the restriction or limiting of species to specific continents or isolated regions) present during the Ordovician Period was replaced during the Silurian Period by a situation where some species were distributed globally. Seafloor topography was muted over large areas of flooded continental platforms, and faunas of shelly invertebrates in different regions were remarkably consistent with one another. This has allowed geologists and stratigraphers to correlate layers of Silurian rock found on different present-day continents.
The geographic summary that follows is based on a global reconstruction specific to the Wenlock Epoch which spans the middle of the Silurian Period.
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Laurentia
Much of North America (including Greenland), northwestern Ireland, Scotland, and the Chukotskiy Peninsula of northeastern Russia belonged to the paleocontinent Laurentia (a name derived from Quebec’s portion of the Canadian Shield). With respect to the present-day Great Lakes and Hudson Bay, Laurentia was rotated clockwise during Wenlock time to fit fully between the latitudes 30° N and 30° S of the paleoequator. The present south shore of Hudson Bay was at the centre of Laurentia, with the Wenlock paleoequator crossing near Southampton Island. The microcontinent of Barentsia, which included Norway’s island of Svalbard, was likely appended to Laurentia off eastern Greenland. Island arcs and highland areas, such as Taconica (a landmass that would become part of eastern North America) and Pearya (a landmass that would become the northern part of Ellesmere Island), rimmed the flooded continent. During the Llandovery and Wenlock epochs, more than 65 percent of Laurentia was flooded by shallow seas.
Baltica
The narrow, north-south Iapetus Ocean still separated Laurentia from another paleocontinent, Baltica, during Wenlock time. The Uralian and Variscan-Hercynian sutures—regions where earlier orogenies (mountain-building events) had welded landmasses together—marked the eastern and southern margins of this paleocontinent. The northern tip of Scandinavia was situated just below the paleoequator during the Wenlock Epoch, but the islands of Novaya Zemlya extended well to the north. The most prominent features were the Caledonian highlands of Norway, although a lowland attributed to the Fennoscandian Shield (rocks of Precambrian origin underlying the present-day Baltic Sea, the Kola Peninsula of Russia, and the countries of Norway, Sweden, and Finland) existed in the vicinity of Finland. Another lowland within Baltica is related to the Sarmatian Shield, in the region between the Vistula and Volga rivers in Poland and adjacent Russia. The microcontinent of Avalonia—its name derived from the Avalon Peninsula of eastern Newfoundland—was appended to Baltica by the end of Ordovician time. It included what are now England, Wales, southeastern Ireland, the Belgian Ardennes, northern France, eastern Newfoundland, part of Nova Scotia, southern New Brunswick, and coastal New England.
Siberia, Kazakhstania, and other continents
Separated from Baltica by the Pleionic Ocean (essentially the northwestern arm of the Paleotethys Sea), the paleocontinent of Siberia assumed an orientation rotated 180° from its present alignment (as recognized by the inverted position of Lake Baikal). A huge Siberian platform sea extended southward. Similarly, Kazakhstania was a neighbouring continent to the east in the same northern middle latitudes. North China (including Manchuria and Korea) and South China (the Yangtze platform) were two separate continents situated in a more equatorial position. In contrast to Siberia and Kazakhstania, most of North and South China were elevated above sea level during Wenlock time.