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Where the Ordovician Period Fits in Geologic Time

The Ordovician Period was the second period of the Paleozoic Era. It followed the Cambrian Period and came before the Silurian. Current international charts place it from about 486.85 to 443.1 million years ago, which is usually rounded to about 487 to 443 million years ago.

The Ordovician is divided into Early, Middle, and Late epochs and seven recognized ages: Tremadocian, Floian, Dapingian, Darriwilian, Sandbian, Katian, and Hirnantian.

The overall theme of the Ordovician is the great diversification of marine life known as the Great Ordovician Biodiversification Event. Then, a severe climate shift and mass extinction brought the period to a close.

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History of Discovery and Naming: The Cambrian-Silurian Controversy

The Ordovician Period was born from one of the longest and most bitter arguments in early geology. During the 1830s, Adam Sedgwick studied the ancient slates, sandstones, and volcanic rocks of North Wales and placed them in his Cambrian System, named after Cambria, the Latin name for Wales. At about the same time, Roderick Murchison studied somewhat younger rocks in southern Wales and along the Welsh border and named them the Silurian System.

At first, their two systems seemed to fit neatly together, with the Cambrian below and the Silurian above. As more rocks were mapped, however, a serious problem appeared. Sedgwick's upper Cambrian rocks overlapped with Murchison's Lower Silurian, meaning that both geologists were claiming many of the same layers and fossils. Neither man wanted to give up part of his system, and the scientific disagreement gradually became a bitter personal feud which lasted until their deaths.

The geology itself made the problem even harder to solve. Many of the disputed rocks had been tightly folded, tilted, overturned, faulted, and repeated. A single sequence could therefore appear several times, sometimes in reverse order. Geologists could not reliably determine which beds were oldest simply by looking at the type of rock or assuming that the layers on top were always younger.

Charles Lapworth began untangling the problem while studying the rocks around Dob's Linn in southern Scotland during the 1870s. He chose this narrow gorge because its steep cliffs exposed a long section of what he called the Moffat Series, named after the river that ran through it, allowing him to trace individual shale, mudstone, grit, and greywacke beds across the landscape. At first, the beds along one cliff appeared to follow a natural sequence. When Lapworth followed the same layers around the gorge, however, he found them repeated in the opposite order.

The map and cross-sections from Lapworth's The Moffat Series, shown below, illustrate this discovery. On one side of the gorge, the rock layers dip steeply in one direction. Farther around the gorge, the same sequence reappears in reverse and dips in the opposite direction. Lapworth realized that part of the sequence had been overturned by folding. The rocks that appeared to lie above one another were not necessarily in their original order (Lapworth, 1878).

This was an important breakthrough, but mapping the folded rocks was only part of the solution. Familiar fossils such as trilobites and brachiopods were uncommon in these beds, so Lapworth focused on small fossils called graptolites. These colonial, planktonic organisms were widespread in ancient oceans, changed rapidly through time, and appeared in distinctive combinations within different layers. This made them excellent index fossils for identifying and matching rocks.

Lapworth carefully recorded which graptolites occurred in each bed. He recognized a sequence of fossil zones within the Moffat Series, including the Glenkiln, Hartfell, and Birkhill shales. These zones confirmed that many apparently different exposures were actually parts of the same folded and repeated sequence. They also revealed major changes in fossil life between the rocks traditionally called Cambrian, Lower Silurian, and Upper Silurian.

In 1879, Lapworth proposed a simple but revolutionary solution: the disputed rocks did not belong entirely to either the Cambrian or Silurian. They represented a separate system between the two. He named it the Ordovician System after the Ordovices, an ancient people who lived in Wales (Harper et al., 2023).

Lapworth had not simply drawn a new boundary between Sedgwick's and Murchison's rocks. He showed that the lower Paleozoic should be divided into three distinct chapters: the Cambrian, Ordovician, and Silurian. His combination of detailed mapping and graptolite index fossils helped settle the controversy and showed paleontologists how fossils could be used to unravel even severely folded and overturned rocks. Acceptance was slow, but the Ordovician was eventually recognized internationally as an official geologic period in 1960.

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What Earth Was Like During the Ordovician Period

The Ordovician world looked very different from Earth today. Gondwana was a huge southern continent that stretched across the South Pole, while smaller continents such as Laurentia, Baltica, and Siberia were separated by broad oceans. The ancient core of North America, Laurentia, sat close to the equator and was partly flooded by warm, shallow seas.

These shallow inland seas created enormous areas of marine habitat. Sunlit seafloors supported brachiopods, bryozoans, trilobites, corals, sponges, crinoids, and mollusks, while graptolites and other plankton drifted through open water. Much of the limestone and shale exposed today around Cincinnati, northern Kentucky, southeastern Indiana, and other parts of North America formed in these seas.

For much of the period, the climate was relatively warm and sea levels were high. Conditions changed near the end of the Ordovician as Gondwana moved across the South Pole and large ice sheets expanded. Sea level fell as water became locked in glacial ice, draining many shallow seas and shrinking the habitats where most marine life lived.

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Life and Major Evolutionary and Environmental Events

The Ordovician was a period of enormous change in the oceans. Early in the period, warm seas covered much of the continents and created vast areas of shallow marine habitat. Over millions of years, marine life became more diverse, food webs grew more complex, and animals spread into new parts of the seafloor and water column. By the end of the period, however, global cooling, glaciation, and falling sea levels brought this long expansion to a sudden and destructive close (Servais et al., 2009).

The Great Ordovician Biodiversification Event and the Rise of Marine Life

One defining event of the period was the Great Ordovician Biodiversification Event, usually shortened to GOBE. Unlike the Cambrian Explosion, which introduced many major animal body plans, the GOBE was mostly about those groups expanding into many new families, genera, species, and lifestyles. It was not one sudden event. Different groups diversified at different times and in different parts of the world over tens of millions of years (Servais et al., 2009; Mangano et al., 2016).

Ordovician seafloors became crowded with brachiopods, bryozoans, trilobites, gastropods, bivalves, corals, sponges, crinoids, and other echinoderms. Reefs grew larger and more complex, built by combinations of algae, sponges, bryozoans, and increasingly important corals. Crinoids raised their feeding arms into the water, while dense communities of brachiopods and bryozoans filtered food from the currents (Mangano et al., 2016).

Life also expanded above the seafloor. Graptolites floated through the open oceans, while tiny plankton supported increasingly complex food webs. Conodont animals became widespread, and straight-shelled nautiloid cephalopods grew into some of the largest and most active predators of the Ordovician seas. Trilobites remained common, but many developed new shapes, spines, fringes, eyes, and feeding adaptations suited to different marine habitats (Mangano et al., 2016).

Fish: The Ordovician also preserves some of the earliest well-known armored jawless fishes. Arandaspids, including forms such as Arandaspis and Sacabambaspis, had broad bony shields covering the head and front of the body, with rows of smaller scales extending along the tail. They had no jaws or paired fins and looked very different from modern fish, but they were among the earliest vertebrates with an extensively mineralized skeleton. Their fossils are mostly found in shallow marine rocks along the margins of Gondwana.

Life was also beginning to move beyond the oceans. Tiny fossil spores show that simple early land plants or their close relatives were beginning to colonize damp environments. These early plants were small and low-growing, but their appearance marked an important early step toward the later development of complex terrestrial ecosystems.

A series of environmental changes may have brought about the GOBE. High sea levels created widespread shallow seas, global temperatures gradually cooled from earlier extremes, and some marine environments became better oxygenated. Changing ocean circulation, expanding plankton communities, shifting continents, and the creation of new habitats also played roles. No single cause explains the entire event (Servais et al., 2009; Mangano et al., 2016).

The Ordovician was also tectonically active. Along eastern Laurentia, an island arc collided with the continent during the Taconic Orogeny, building mountains and sending volcanic ash into surrounding seas. Around the Middle Ordovician, a major asteroid breakup also produced an unusual increase in meteorites and extraterrestrial dust reaching Earth. Whether that event encouraged diversification remains debated (Servais et al., 2009).

The End-Ordovician Mass Extinction

The Ordovician ended with a dramatic reversal. Gondwana had fully moved over the South Pole, and large ice sheets spread across parts of the supercontinent. Global temperatures fell and so much water became trapped in glaciers that sea level dropped sharply. Vast shallow seas drained away, removing many of the habitats in which Ordovician marine life had flourished (Servais et al., 2009).

The extinction occurred in at least two major pulses. The first was closely connected to rapid cooling, glaciation, and the loss of shallow marine environments. Brachiopods, bryozoans, trilobites, corals, graptolites, conodonts, echinoderms, and many other groups suffered heavy losses as temperature zones shifted and continental seas disappeared.

A second wave of extinction followed as the climate and oceans changed again. When the glaciers began to retreat, sea level rose, ocean circulation reorganized, and low-oxygen or even sulfidic waters expanded through some marine basins. Animals that had survived the initial cooling were forced to adjust to another rapid environmental shift.

The End-Ordovician extinction brought the period to a close and is recognized as the first of the traditional "Big Five" mass extinctions. An estimated 85 percent of marine species disappeared, including many brachiopods, bryozoans, trilobites, graptolites, corals, and conodonts. Although surviving groups recovered and diversified during the Silurian, the rich marine ecosystems created during the GOBE were permanently reshaped.

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Ordovician Trilobites

Ordovician trilobites include many distinctive forms, from filter-feeding lace-collar trilobites to species known for rare soft-part preservation. More than 800 genera and about 5,460 valid species have been recorded from the Ordovician (Adrain et al., 2004; Adrain, 2013). During this period, trilobites diversified into many specialized body plans with unusual head shields, different eye arrangements, long spines, and adaptations for feeding or moving across the seafloor and swimming in the water column.

Below are links to guides for several well-known Ordovician trilobites from North America.

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  Isotelus

  The giant smooth-shelled trilobite of the Ordovician seas.

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  Cryptolithus

  The lace-collar trilobite, known for its wide perforated cephalon.

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  Triarthrus

  Triarthrus eatoni, famous for pyritized soft-part preservation.

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Famous Fossil Sites from the Ordovician Period

Ordovician rocks are exposed around the world, and many sites preserve very different parts of the period's story. The Fezouata Shale of Morocco contains exceptionally preserved animals, including soft-bodied forms that show some Cambrian-style groups survived well into the Ordovician. Beecher's Trilobite Bed in New York is famous for pyritized Triarthrus trilobites with preserved legs, antennae, gills, mouthparts, and even eggs.

The Soom Shale of South Africa preserves soft tissues and delicate animals from cold, low-oxygen seas near the end of the period. Anticosti Island in Quebec contains an unusually complete marine record across the Ordovician-Silurian boundary, making it important for studying the mass extinction and recovery.

For fossil collectors, the Cincinnati Arch region is one of the best-known Ordovician areas in North America. Upper Ordovician limestones and shales exposed around Cincinnati, northern Kentucky, southeastern Indiana, and southwestern Ohio contain abundant brachiopods, bryozoans, corals, gastropods, crinoid pieces, cephalopods, and trilobites. Many of these fossils are preserved in rocks that formed in warm, shallow seas near the equator.

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References / Works Cited

Adrain, J. M., Edgecombe, G. D., Zhou, Z., Fortey, R. A., Hammer, O., Laurie, J. R., McCormick, T., Owen, A. W., Waisfeld, B. G., Webby, B. D., & Westrop, S. R. (2004). Trilobites. In B. D. Webby, F. Paris, M. L. Droser, & I. G. Percival (Eds.), The Great Ordovician Biodiversification Event, pp. 231-254. Columbia University Press. https://doi.org/10.7312/webb12678-025

Adrain, J. M. (2013). A synopsis of Ordovician trilobite distribution and diversity. In D. A. T. Harper & T. Servais (Eds.), Early Palaeozoic Biogeography and Palaeogeography. Geological Society, London, Memoirs, 38, 297-336. https://doi.org/10.1144/M38.20

Finnegan, S., Heim, N. A., Peters, S. E., & Fischer, W. W. (2012). Climate change and the selective signature of the Late Ordovician mass extinction. Proceedings of the National Academy of Sciences, 109(18), 6829-6834. https://doi.org/10.1073/pnas.1117039109

Gradstein, F. M., Ogg, J. G., Schmitz, M. D., & Ogg, G. M. (Eds.). (2020). Geologic Time Scale 2020. Elsevier.

International Commission on Stratigraphy. (2024). International Chronostratigraphic Chart, v2024/12. stratigraphy.org/chart

Lapworth, C. (1879). On the Tripartite Classification of the Lower Palaeozoic Rocks. Geological Magazine, New Series, 6, 1-15.

Lapworth, C. (1879-1880). On the Geological Distribution of the Rhabdophora. The Annals and Magazine of Natural History: Zoology, Botany, and Geology, vol. 3, pp. 245-257 and 449-455;vol. 4, pp. 333-341 and 423-431;vol. 5, pp. 45-62, 273-285, and 359-369;vol. 6, pp. 16-29 and 185-207. View original publication.

Lapworth, C. (1878). The Moffat Series. Quarterly Journal of the Geological Society of London, 34, 240-346. View original publication.

Mangano, M. G., Buatois, L. A., Wilson, M., & Droser, M. (2016). The Great Ordovician Biodiversification Event. In M. G. Mángano & L. A. Buatois (Eds.), The Trace-Fossil Record of Major Evolutionary Events, Topics in Geobiology, Vol. 39, pp. 127-156. Springer. https://doi.org/10.1007/978-94-017-9600-2_4

Servais, T., Harper, D. A. T., Li, J., Munnecke, A., Owen, A. W., & Sheehan, P. M. (2009). Understanding the Great Ordovician Biodiversification Event (GOBE): Influences of paleogeography, paleoclimate, and paleoecology. GSA Today, 19(4/5), 4-10. https://doi.org/10.1130/GSATG37A.1

Sheehan, P. M. (2001). The Late Ordovician Mass Extinction. Annual Review of Earth and Planetary Sciences, 29, 331-364. https://doi.org/10.1146/annurev.earth.29.1.331

Stigall, A. L., Freeman, R. L., Edwards, C. T., & Rasmussen, C. M. O. (2020). A multidisciplinary perspective on the Great Ordovician Biodiversification Event and the development of the early Paleozoic world. Palaeogeography, Palaeoclimatology, Palaeoecology, 543, 109521. https://doi.org/10.1016/j.palaeo.2019.109521

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