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Where the Silurian Period Fits in Geologic Time

The Silurian Period was the third period of the Paleozoic Era. It followed the Ordovician Period and came before the Devonian. The current International Chronostratigraphic Chart places it from about 443.1 to 419.62 million years ago.

The Silurian is divided into four series or epochs: the Llandovery, Wenlock, Ludlow, and Pridoli. The first three contain seven named stages: Rhuddanian, Aeronian, Telychian, Sheinwoodian, Homerian, Gorstian, and Ludfordian. The Pridoli is not currently divided into globally ratified stages.

The broad theme of the Silurian is recovery and expansion. Marine ecosystems rebuilt after the end-Ordovician extinction, reefs spread through tropical seas, jawed vertebrates became increasingly important, and simple plants and arthropods occupied damp environments on land. The transition into the Devonian was gradual rather than marked by another Big Five mass extinction.

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History of Discovery and Naming: Roderick Murchison and the Silurian System

The Silurian was one of the earliest formally named systems of ancient rocks. During the 1830s, Scottish geologist Roderick Impey Murchison mapped fossil-bearing limestones, shales, sandstones, and volcanic rocks across the borderlands of Wales and England. He found that the fossils in these strata formed a recognizable sequence that differed from the younger Old Red Sandstone above them.

Murchison announced the name Silurian System in July 1835. He chose the name from the Silures, an Iron Age people who had lived in the Welsh Borderlands. At the time, geologists were beginning to divide Earth's history into large systems defined not only by rock type, but also by the fossils contained within those rocks.

His research became the enormous two-volume work The Silurian System, published in 1839. The book combined geological maps, measured sections, landscape views, and detailed fossil illustrations. It helped establish the use of fossil succession for identifying and correlating ancient rocks across broad regions. Murchison's wife, Charlotte Murchison, also accompanied him in the field, made sketches, collected fossils, and contributed to the geological work behind his publications.

The original Silurian System was much larger than the modern Silurian Period. Murchison's lower Silurian rocks overlapped with the upper part of Adam Sedgwick's Cambrian System, producing the long Cambrian-Silurian controversy. In 1879, Charles Lapworth resolved much of the overlap by creating the Ordovician System between the Cambrian and Silurian. The modern Silurian therefore represents only the upper part of what Murchison originally called Silurian, yet even after nearly two centuries of revision, the name Silurian remains a lasting part of Murchison's geological legacy.

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What Earth Was Like During the Silurian Period

The Silurian world was divided among several large continents and smaller landmasses. Gondwana stretched across much of the Southern Hemisphere, while Laurentia, the ancient core of North America, remained near the equator. Baltica and Avalonia moved closer to Laurentia as the Iapetus Ocean narrowed between them.

These continental movements were slowly assembling a larger northern landmass. Collisions around the closing Iapetus Ocean produced mountain building associated with the Caledonian and related orogenies. The remains of these ancient mountain belts can now be traced through parts of eastern North America, Greenland, Ireland, Scotland, and Scandinavia.

At the beginning of the Silurian, Earth was recovering from the intense glaciation that ended the Ordovician. As ice sheets retreated, sea level rose and shallow seas again spread across broad continental platforms. Warm, clear tropical waters encouraged the development of extensive carbonate platforms and large reef systems.

Silurian reefs were built by communities rather than one single kind of organism. Stromatoporoid sponges, tabulate corals, rugose corals, algae, and other reef dwellers formed rigid structures that provided shelter and feeding surfaces for brachiopods, crinoids, trilobites, mollusks, and many smaller animals. Some thick Silurian limestone deposits around the Great Lakes and Gotland formed in these tropical marine settings.

The climate was generally warmer than during the end-Ordovician ice age, but it was not completely stable. Sea level rose and fell repeatedly, ocean circulation changed, and oxygen-poor water expanded into some basins. Several intervals of rapid environmental change produced major shifts in marine communities and left strong chemical signals in Silurian rocks.

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Life and Major Evolutionary and Environmental Events

The Silurian began in the aftermath of one of the largest mass extinctions in Earth history. Although many Ordovician species were gone, most major marine animal groups survived. During the next 23 million years, those survivors rebuilt complex ecosystems in expanding shallow seas. At the same time, several major evolutionary changes were unfolding in fishes and on land.

Recovery After the End-Ordovician Mass Extinction

The first Silurian communities were shaped by survivors of the end-Ordovician extinction. Brachiopods, bryozoans, crinoids, corals, trilobites, mollusks, graptolites, and conodonts gradually diversified again. Recovery was not instantaneous. Some groups rebounded quickly, while others required millions of years to regain ecological variety.

As sea levels rose, shallow marine habitats expanded across Laurentia, Baltica, and other continents. Filter feeders covered parts of the seafloor, crinoid meadows lifted feeding arms into the currents, and reef communities became larger and more widespread. Reefs, open seafloors, lagoons, tidal flats, and deeper basins created many different habitats.

Graptolites remained important planktonic animals and are among the most useful fossils for dating Silurian rocks. Their species changed rapidly through time and were distributed across broad ocean regions. Conodont elements, the tooth-like feeding structures of small eel-shaped vertebrates, also provide a detailed record for dividing and correlating Silurian strata.

The Rise of Jawed Fishes

The Silurian is one of the most important periods in the history of vertebrates. Jawless fishes were still widespread, including armored forms with bony head shields and scales. However, fishes with jaws began leaving a much clearer fossil record and diversified into several major branches.

Tiny tooth whorls of Qianodus duplicis from China provide direct evidence of toothed jawed vertebrates about 439 million years ago. These fossils pushed the known history of gnathostome teeth much deeper into the Early Silurian and show that important features of jaw and tooth development had already evolved.

Even more remarkable fossils come from the Early Silurian Chongqing Lagerstatte of China, about 436 million years old. Complete bodies of Xiushanosteus mirabilis and Shenacanthus vermiformis reveal that jawed fishes had already developed unexpectedly diverse combinations of armor, fin spines, jaws, and body shapes. These discoveries replaced a fossil record once known mainly from scattered scales and spines with articulated animals that could be reconstructed in detail.

A 2026 study described Eosteus chongqingensis, a small, near-complete fish from the same Early Silurian deposits. It is the oldest articulated bony fish yet known and shows that the lineage leading to the enormous modern diversity of bony fishes was already present by about 436 million years ago.

By the Late Silurian, placoderms, early cartilaginous fishes, and early bony fishes were more varied and widespread. Some remained only a few centimeters long, while others were becoming much larger predators. This Silurian radiation helped prepare the way for the Devonian, often called the Age of Fishes.

Plants and Animals Expand onto Land

Silurian land would have looked nearly empty compared with a modern landscape. There were no trees, flowers, grasses, forests, or vertebrate land animals. Most exposed surfaces were bare rock, sand, mud, or thin microbial coverings. Even so, the foundations of later terrestrial ecosystems were taking shape.

The best-known Silurian land plant is Cooksonia. It had slender, leafless stems that repeatedly divided into two branches, with small spore-producing structures at the tips. Fossils preserve conducting tissue capable of moving water through the plant, making Cooksonia one of the earliest well-known vascular plants.

Near the end of the Silurian, these low-growing plants shared the landscape with Prototaxites, a mysterious trunk-like organism that appeared long before true trees. Later Devonian examples grew more than 26 feet tall, towering over the surrounding vegetation. Once interpreted as a primitive tree and later as a giant fungus, Prototaxites may instead represent an entirely extinct branch of complex life with no close modern equivalent.

These plants remained only a few centimeters tall and probably grew in damp environments near streams, ponds, tidal flats, and wet lowlands. They lacked true roots and leaves in the modern sense, but their vascular tissues, upright stems, and airborne spores were important adaptations for life out of water.

Arthropods were also moving onto land. Late Silurian rocks in Britain preserve remains of centipede-like myriapods and trigonotarbids, extinct arachnids related to spiders and other chelicerates. Their presence shows that small terrestrial food webs were developing among the earliest plants, fungi, microbial mats, and decaying organic material.

Silurian Environmental Crises

The Silurian did not end with a Big Five mass extinction, but the period was interrupted by several major biological and environmental crises. The best known are the Ireviken, Mulde, and Lau events. Each occurred at a different time and affected marine groups in different ways.

Conodonts and graptolites suffered especially severe losses during some of these events, while trilobites, brachiopods, reef organisms, and other animals also changed. The events are associated with large shifts in carbon isotopes, indicating major changes in the global carbon cycle.

The exact causes remain debated, but rapid climate change, sea-level movement, changes in nutrient supply, and the spread or retreat of oxygen-poor waters probably interacted. Silurian reefs repeatedly expanded during favorable intervals and then declined when environmental conditions changed.

These crises show that the Silurian was not simply a long, calm recovery between two better-known periods. It was a dynamic interval in which climate, oceans, and ecosystems repeatedly reorganized. Many groups that survived these disruptions later became major parts of Devonian marine and terrestrial ecosystems.

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Silurian Trilobites and Sea Scorpions

Trilobites survived the end-Ordovician extinction and remained important members of Silurian marine communities. Their global diversity was lower than during the Ordovician, but many distinctive forms were common on local seafloors. Familiar Silurian genera include Dalmanites, Encrinurus, Calymene, Gravicalymene, and Lichas.

Silurian trilobites occupied muddy bottoms, carbonate platforms, reef margins, and deeper marine settings. Some had broad, smooth bodies suited for moving through soft sediment. Others developed elaborate spines, strongly sculptured shells, or large eyes. Molted shell pieces are much more common than complete enrolled or outstretched specimens.

Eurypterids, commonly called sea scorpions, became some of the most recognizable arthropods of the Silurian. They were aquatic chelicerates related to arachnids and horseshoe crabs. Despite their common name, not all lived in open marine water. Different species occupied marine shorelines, lagoons, estuaries, brackish environments, and possibly freshwater habitats.

Many eurypterids were modest in size and probably fed on small animals or organic material along the bottom. Others, including large pterygotids, were active predators with enlarged grasping appendages. Their streamlined bodies and paddle-like limbs made them effective swimmers. The Bertie Group of New York and Ontario is especially famous for beautifully preserved Silurian eurypterids.

Trilobites and eurypterids represent two different arthropod stories. Trilobites were ancient seafloor animals that had already existed for more than 100 million years, while eurypterids were expanding into a variety of coastal and inland aquatic habitats. Both continued into later Paleozoic periods, but eurypterids reached some of their greatest diversity during the Silurian.

For more information about Eurypterids, visit my Eurypterid Page.

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Famous Fossil Sites from the Silurian Period

Silurian rocks occur on every continent, and several sites preserve exceptional records of life that would normally disappear. The Herefordshire Lagerstatte in England contains small marine animals preserved three-dimensionally inside limestone concretions. Digital reconstructions have revealed soft-bodied arthropods, worms, mollusks, echinoderms, sponges, and other animals with delicate anatomy.

The Waukesha Biota of Wisconsin is an Early Silurian deposit with unusually preserved soft tissues and lightly mineralized animals. It includes worms, arthropods, conodont animals, early chelicerate relatives, and organisms that are rare or unknown from ordinary shell beds. The site offers a view of marine life soon after the end-Ordovician extinction.

The Rochester Shale of western New York and southern Ontario is one of North America's most famous Middle Silurian fossil formations. Its shale and limestone preserve diverse marine communities that lived about 430 million years ago. The formation is especially known for complete trilobites, including Dalmanites limulurus and the large Arctinurus boltoni, along with crinoids, cystoids, brachiopods, bryozoans, corals, and mollusks.

Many of the finest specimens came from the Middleport Quarry, also known as the Caleb Quarry. Research by Carlton E. Brett and his students established its stratigraphy and documented important echinoderm Lagerstatten and Arctinurus beds. The Smithsonian excavated the site in 1990, and the Middleport Fossil Recovery Project later documented more than 120 fossil species during over two decades of organized collecting and research. Once internationally famous for its exceptionally preserved fossils, the quarry is now privately operated and closed to public collecting.

The Chongqing Lagerstatte of China has transformed the history of vertebrates. Complete Early Silurian jawed fishes from this deposit show that placoderm-like fishes, cartilaginous-fish relatives, and bony-fish relatives had already diversified by about 436 million years ago.

The Bertie Group of New York and Ontario is world-famous for eurypterids, including Eurypterus and related forms. Fine-grained dolostones formed in restricted, shallow-water environments where entire molts and bodies could sometimes be preserved. These rocks produced many museum specimens and helped make sea scorpions icons of the Silurian.

The island of Gotland in Sweden preserves a long Silurian sequence rich in reefs, corals, stromatoporoids, brachiopods, trilobites, and crinoids. Its coastal exposures and detailed fossil record make it one of the classic regions for studying Silurian reefs, sea-level changes, and environmental events.

Other important areas include Lesmahagow in Scotland, with fishes and eurypterids; the Welsh Borderlands where Murchison established the Silurian System; Anticosti Island in Quebec; and widespread Silurian limestones and dolostones across Ohio, Indiana, Michigan, Wisconsin, Illinois, and Ontario.

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References / Works Cited

Andreev, P. S., Sansom, I. J., Li, Q., Zhao, W., Wang, J., Wang, C., Peng, L., Jia, L., Qiao, T., & Zhu, M. (2022). The oldest gnathostome teeth. Nature, 609, 964-968. https://doi.org/10.1038/s41586-022-05166-2

Edwards, D., Bassett, M. G., & Rogerson, E. C. W. (1979). The earliest vascular land plants: continuing the search for proof. Lethaia, 12, 313-324. https://doi.org/10.1111/j.1502-3931.1979.tb01017.x

Edwards, D., Davies, K. L., & Axe, L. (1992). A vascular conducting strand in the early land plant Cooksonia. Nature, 357, 683-685. https://doi.org/10.1038/357683a0

International Commission on Stratigraphy. (2026). International Chronostratigraphic Chart, v2026/06. stratigraphy.org/chart

Jeram, A. J., Selden, P. A., & Edwards, D. (1990). Land animals in the Silurian: arachnids and myriapods from Shropshire, England. Science, 250, 658-661. https://doi.org/10.1126/science.250.4981.658

Murchison, R. I. (1839). The Silurian System. John Murray. View original publication.

Peach, B. N. (1882). On some fossil myriapods from the Lower Old Red Sandstone of Forfarshire. Proceedings of the Royal Physical Society of Edinburgh, 7, 177-188. https://www.biodiversitylibrary.org/page/16121555

Siveter, D. J., Briggs, D. E. G., Siveter, D. J., & Sutton, M. D. (2020). The Herefordshire Lagerstatte: fleshing out Silurian marine life. Journal of the Geological Society, 177, 1-13. https://doi.org/10.1144/jgs2019-110

Wendruff, A. J., Babcock, L. E., Kluessendorf, J., & Mikulic, D. G. (2020). Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA. Palaeogeography, Palaeoclimatology, Palaeoecology, 546, 109631. https://doi.org/10.1016/j.palaeo.2020.109631

Zhu, Y., Li, Q., Lu, J., Chen, Y., Wang, J., Gai, Z., Zhao, W., Wei, G., Yu, Y., Ahlberg, P. E., & Zhu, M. (2022). The oldest complete jawed vertebrates from the early Silurian of China. Nature, 609, 954-958. https://doi.org/10.1038/s41586-022-05136-8

Zhu, Y., Chen, Y., Li, Q., et al. (2026). The oldest articulated bony fish from the early Silurian period. Nature, 651, 128-134. https://doi.org/10.1038/s41586-026-10125-2

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