Ordovician
The Ordovician period was a vibrant chapter in Earth's history, a time when life truly exploded in the oceans, setting the stage for much of the biodiversity we see today. From the first reefs to the earliest tentative steps of plants onto land, this era witnessed profound evolutionary leaps. But this dynamic world eventually faced one of the most severe mass extinctions our planet has ever known—let's journey back to discover its story. The Ordovician experienced an unprecedented diversification of marine life, an event often called the Great Ordovician Biodiversification. During this period, Earth's continents were constantly in motion, influencing ocean currents and leading to significant mountain-building events. The Ordovician concluded with a severe global ice age and one of Earth's five largest extinction events, drastically reshaping the course of life.
AI Summary
The Ordovician period was a vibrant chapter in Earth's history, a time when life truly exploded in the oceans, setting the stage for much of the biodiversity we see today. From the first reefs to the earliest tentative steps of plants onto land, this era witnessed profound evolutionary leaps. But this dynamic world eventually faced one of the most severe mass extinctions our planet has ever known—let's journey back to discover its story.
- The Ordovician experienced an unprecedented diversification of marine life, an event often called the Great Ordovician Biodiversification.
- During this period, Earth's continents were constantly in motion, influencing ocean currents and leading to significant mountain-building events.
- The Ordovician concluded with a severe global ice age and one of Earth's five largest extinction events, drastically reshaping the course of life.
The Age of Marine Marvels
Spanning over 43 million years, from about 485 to 443 million years ago, the Ordovician period holds a pivotal place in Earth's deep past. It's the second act of the Paleozoic Era, nestled between the famous Cambrian 'explosion' and the Silurian period that followed. Imagine a world profoundly different from our own, dominated by vast, shallow seas teaming with bizarre and wonderful creatures.
This period gets its name from the 'Ordovices,' an ancient Celtic tribe that once inhabited parts of North Wales. It was named in 1879 by British geologist Charles Lapworth, who settled a heated geological debate by recognizing that a unique set of rocks and fossils deserved its own distinct time interval—a decision finally solidified by international approval in 1960.
A Burst of Life, a Cold End
While the Cambrian is famed for the initial burst of complex life, the Ordovician saw life truly diversify and flourish. This era was characterized by the Great Ordovician Biodiversification Event, where marine ecosystems became incredibly complex and rich. Invertebrates like molluscs and arthropods ruled the oceans, and we see the very first hints of land plants beginning to establish themselves on Earth's barren continents.
Even fish, our world's first true vertebrates, continued their evolutionary journey, with some evidence suggesting the earliest jawed fish might have appeared late in the period. Intriguingly, this time also saw a surge in meteorite impacts—around 100 times more frequent than today—possibly linked to a temporary ring system forming around Earth. It was a dynamic, sometimes violent, but ultimately transformative era.
Mapping Geologic Time
Just like we divide our calendar into months and weeks, geologists carve Earth's immense history into smaller, manageable chunks. The Ordovician Period itself is divided into three major series: the Early, Middle, and Late Ordovician. These are further broken down into even finer stages, each named after a specific location where their characteristic rock layers were first identified.
Series/Epoch Stage/Age Lower Boundary (Ma) Upper/Late Ordovician Hirnantian 445.2 ± 0.9 Katian 452.8 ± 0.7 Sandbian 458.2 ± 0.7 Middle Ordovician Darriwilian 469.4 ± 0.9 Dapingian 471.3 ± 1.4 Lower/Early Ordovician Floian 477.1 ± 1.2 Tremadocian 486.85 ± 1.5
A World on the Move
Imagine Earth's continents during the Ordovician—they looked drastically different! The southern continents were clumped together into the supercontinent Gondwana, stretching from near the equator all the way to the South Pole. Meanwhile, smaller continents like Laurentia (proto-North America), Siberia, and Baltica (northern Europe) drifted independently, separated by vast stretches of ocean. Over half the globe was covered by the enormous Panthalassic Ocean.
This continental dance meant that each landmass developed its own distinct communities of organisms. A microcontinent called Avalonia, which would eventually become parts of modern-day Europe and North America, began to rift away from Gondwana and slowly drifted north, opening up the Rheic Ocean in its wake. It was a time of constant tectonic shifts and evolving geographies.
This wasn't just quiet drifting; the Ordovician was a period of extensive tectonism and volcanism. Mountain-building, known as orogenesis, was happening vigorously, though not primarily from continents crashing head-on. Instead, mountains rose as volcanic island arcs and microcontinents accreted—or merged—along active continental margins. It was a global reorganization of tectonic plates.
For instance, the Taconic Orogeny, which had begun in the Cambrian, continued into the Ordovician. This massive mountain-building event involved at least two volcanic island arcs colliding with Laurentia, slowly forming the ancestral Appalachian Mountains in North America. Elsewhere, like along South China, similar accretion events reshaped the land.
A particularly dramatic event was the Millburg/Big Bentonite ash fall around 454 million years ago—the largest volcanic ash fall in the last 590 million years! Yet, despite an estimated volume of over 1,140 cubic kilometers of dense rock equivalent, this immense geological upheaval appears to have had surprisingly little impact on the flourishing life of the Ordovician oceans.
Meanwhile, Gondwana's margins were also highly active. Intense tectonic activity in what is now Spain and Turkey reshaped its northwest. On its eastern flank, island arcs collided with ancient Australia, giving rise to the Benambran Orogeny. These collisions and subductions were dynamic forces, constantly sculpting the Earth's surface.
As the period drew to a close, Gondwana began a slow, inexorable drift towards the South Pole. This monumental movement would have profound consequences, setting the stage for a global climate shift and one of the most significant extinction events in Earth's history.
The Calcite Seas
The chemistry of the Ordovician oceans played a crucial role in shaping marine life. This was a 'calcite sea' period, meaning that low-magnesium calcite was the predominant form of calcium carbonate precipitated from seawater. This favored organisms that built their shells and skeletons from calcite, making calcitic hardgrounds, ooids, and cements very common.
Unlike the Cambrian, where microbial processes largely dominated calcite production, the Ordovician marked a shift. Animals and large algae became the primary architects of calcareous material in marine deposits. This biological influence dramatically changed the landscape of the seafloor and the types of fossils preserved.
From Greenhouse to Icehouse
The early Ordovician climate was quite hot, characterized by intense greenhouse conditions with sea surface temperatures comparable to modern extremes. Carbon dioxide levels in the atmosphere were extremely high. But as the period progressed, Earth began to cool, slowly transitioning into a more temperate climate by the Middle Ordovician.
This cooling trend culminated in the Late Ordovician, when Gondwana's polar drift triggered the Hirnantian glaciation—a true ice age. Sea levels, which had been at their highest of the Paleozoic for much of the period, began to fall steadily as vast ice caps formed on the southern continents. These dramatic climate shifts profoundly impacted marine ecosystems worldwide.
For much of the Ordovician, vast shallow seas covered continental shelves across the globe, including large portions of Gondwana, North America, and Europe. These clear, warm, shallow waters were perfect nurseries for marine life, especially for organisms that built shells and hard parts from calcium carbonate, fueling the incredible biodiversity of the era.
The Flourishing Oceans
The Ordovician was a time of unparalleled biological innovation in the oceans, where life not only recovered from the Cambrian but exploded in diversity and ecological complexity. However, this vibrant era was tragically capped by a series of mass extinction events that severely impacted many groups, particularly planktonic organisms and those sensitive to environmental changes.
The Marine Explosion
The Great Ordovician Biodiversification Event (GOBE) was a game-changer. Marine faunal genera increased fourfold, leading to 12% of all known Phanerozoic marine fauna originating during this time. The ecological system reached a new level of complexity, characterized by tiered communities of suspension feeders and short food chains. Many animals even underwent a miniaturization process compared to their Cambrian ancestors.
This period saw a significant shift in dominant life forms. The trilobites and inarticulate brachiopods of the Cambrian were largely succeeded by articulate brachiopods, cephalopods, and crinoids, which would dominate the remainder of the Paleozoic. Articulate brachiopods, with their more complex shells, were particularly successful, thriving in nearly every marine environment.
The separation of continents created 'provincialism,' with distinct trilobite faunas evolving on Laurentia, Baltica, and Gondwana. However, as the Ordovician progressed and continents began to converge, marine taxa started dispersing more widely, making faunas less provincial by the late Ordovician, though unique regional characteristics still persisted.
Trilobites, those iconic arthropods, were incredibly diverse and rapidly diversified. Ordovician trilobites were far more varied than their Cambrian predecessors, developing bizarre spines, nodules, shovel-like snouts for burrowing, and even long eyestalks like Asaphus kowalewski to detect predators. Some, like the trinucleids, evolved broad, pitted head shields, while others became streamlined swimmers.
Beyond trilobites, reef-forming corals made their grand entrance, creating complex habitats that supported even more life. Molluscs, including bivalves, gastropods, and nautiloid cephalopods, became common and diversified, with cephalopods conquering almost all marine environments. Graptolites, a type of colonial animal, thrived in the open oceans, and new forms of echinoderms like cystoids and crinoids emerged.
While the first true vertebrates (fish) are now thought to have appeared earlier in the Cambrian, the Ordovician saw their continued evolution. Crucially, the first jawed fish, or gnathostomes, may have made their debut late in the period, marking a monumental evolutionary step. This innovation would eventually lead to the vast diversity of fish and, ultimately, all land vertebrates, including us.
The Middle Ordovician also witnessed a dramatic 'Bioerosion Revolution.' Organisms started actively boring into hard substrates like shells and carbonate hardgrounds, leaving behind characteristic trace fossils such as Trypanites and Petroxestes. This marked a significant increase in the intensity and diversity of biological activity altering the seafloor.
Life's First Steps on Land
While the oceans teemed with life, the land remained largely barren throughout the Cambrian. But in the Ordovician, a quiet revolution began on the margins. Green algae, common in the seas, are thought to be the ancestors of terrestrial plants. Tiny, non-vascular forms, perhaps resembling modern liverworts, made their initial, tentative colonization of damp shorelines.
Fossilized spores typical of bryophytes have been found in Ordovician rocks, providing direct evidence of these early pioneers. Crucially, early land fungi, like arbuscular mycorrhiza, played a vital symbiotic role, making mineral nutrients available to these nascent land plants. This partnership was essential for life to gain its first foothold on Earth's continents, paving the way for future forests and ecosystems.
The Great Ordovician Extinction
The vibrant Ordovician Period drew to a close with a series of devastating extinction events, collectively the second-largest mass extinction in Earth's history in terms of the percentage of genera lost. Only the Permian-Triassic extinction event, millions of years later, surpassed its severity. Nearly half of all marine genera vanished forever.
These extinctions, occurring around 447 to 444 million years ago, directly mark the boundary with the subsequent Silurian Period. They are primarily attributed to the onset of the Hirnantian glaciation, a massive ice age triggered by Gondwana's drift over the South Pole. This drastic shift from stable greenhouse conditions to an icehouse world dramatically altered global climate and sea levels.
Article
Ordovician
The Ordovician ( or-də-VISH-ee-ən, -doh-, -VISH-ən) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 43.75 million years from the end of the Cambrian Period 486.85 Ma (million years ago) to the start of the Silurian Period 443.1 Ma.
The Ordovician, named after the Welsh 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 North Wales in 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 had 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, with members of the latter group probably starting their establishment on land during this time, becoming fully established by the Devonian. The first land plants are known from this period. 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. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today in a period known as the Ordovician meteor event. It has been theorized that this increase in impacts may originate from a ring system that formed around Earth at the time.
Subdivisions
Ordovician
In 2008, the ICS erected a formal international system of subdivisions for the Ordovician Period and System. Pre-existing Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North-Gondwanan regional stratigraphic schemes are also used locally.
<table><thead><tr><th>Series/epoch</th><th>Stage/age</th><th>Lower boundary</th></tr></thead><tbody><tr><td>Upper/Late Ordovician</td><td>Hirnantian</td><td>(445.2 ± 0.9 Ma)</td></tr><tr><td>Katian</td><td>(452.8 ± 0.7 Ma)</td></tr><tr><td>Sandbian</td><td>(458.2 ± 0.7 Ma)</td></tr><tr><td>Middle Ordovician</td><td>Darriwilian</td><td>(469.4 ± 0.9 Ma)</td></tr><tr><td>Dapingian</td><td>(471.3 ± 1.4 Ma)</td></tr><tr><td>Lower/Early Ordovician</td><td>Floian</td><td>(477.1 ± 1.2 Ma)</td></tr><tr><td>Tremadocian</td><td>(486.85 ± 1.5 Ma)</td></tr></tbody></table>
Paleogeography and tectonics
Ordovician
Paleogeographic map of the Earth in the early Ordovician, 480 million years ago
Paleogeographic map of the Earth in the middle Ordovician, 470 million years ago
Paleogeographic map of the Earth in the late Ordovician, 450 million years ago
During the Ordovician, the southern continents were assembled into Gondwana, which reached from north of the equator to the South Pole. The Panthalassic Ocean, centered in the northern hemisphere, covered over half the globe. At the start of the period, the continents of Laurentia (in present-day North America), Siberia, and Baltica (present-day northern Europe) were separated from Gondwana by over 5,000 kilometres (3,100 mi) of ocean. These smaller continents were also sufficiently widely separated from each other to develop distinct communities of benthic organisms. The small continent of Avalonia had just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. Avalonia collided with Baltica towards the end of Ordovician.
Other geographic features of the Ordovician world included the Tornquist Sea, which separated Avalonia from Baltica; the Aegir Ocean, which separated Baltica from Siberia; and an oceanic area between Siberia, Baltica, and Gondwana which expanded to become the Paleoasian Ocean in Carboniferous time. The Mongol-Okhotsk Ocean formed a deep embayment between Siberia and the Central Mongolian terranes. Most of the terranes of central Asia were part of an equatorial archipelago whose geometry is poorly constrained by the available evidence.
The period was one of extensive, widespread tectonism and volcanism. However, orogenesis (mountain-building) was not primarily due to continent-continent collisions. Instead, mountains arose along active continental margins during accretion of arc terranes or ribbon microcontinents. Accretion of new crust was limited to the Iapetus margin of Laurentia; elsewhere, the pattern was of rifting in back-arc basins followed by remerger. This reflected episodic switching from extension to compression. The initiation of new subduction reflected a global reorganization of tectonic plates centered on the amalgamation of Gondwana.
The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. This continued into the Ordovician, when at least two volcanic island arcs collided with Laurentia to form the Appalachian Mountains. Laurentia was otherwise tectonically stable. An island arc accreted to South China during the period, while subduction along north China (Sulinheer) resulted in the emplacement of ophiolites.
The ash fall of the Millburg/Big Bentonite bed, at about 454 Ma, was the largest in the last 590 million years. This had a dense rock equivalent volume of as much as 1,140 cubic kilometres (270 cu mi). Remarkably, this appears to have had little impact on life.
There was vigorous tectonic activity along northwest margin of Gondwana during the Floian, 478 Ma, recorded in the Central Iberian Zone of Spain. The activity reached as far as Turkey by the end of Ordovician. The opposite margin of Gondwana, in Australia, faced a set of island arcs. The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia. Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma. This involved significant back arc rifting. The interior of Gondwana was tectonically quiet until the Triassic.
Towards the end of the Ordovician, Gondwana began to drift across the South Pole; this contributed to the Hirnantian glaciation and the associated extinction event.
Ordovician meteor event
The Ordovician meteor event is a proposed shower of meteors that occurred during the Middle Ordovician Epoch, about 467.5 ± 0.28 million years ago, due to the break-up of the L chondrite parent body. It is not associated with any major extinction event. A 2024 study found that craters from this event cluster in a distinct band around the Earth, and that the breakup of the parent body may have formed a ring system for a period of about 40 million years, with frequent falling debris causing these craters.
Geochemistry
Ordovician
External mold of the Ordovician bivalve Anomalodonta gigantea showing that the original aragonite shell dissolved on the sea floor, leaving a cemented mold for biological encrustation (Waynesville Formation of Franklin County, Indiana).
The Ordovician was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons. Biogenic aragonite, like that composing the shells of most molluscs, dissolved rapidly on the sea floor after death.
Unlike Cambrian times, when calcite production was dominated by microbial and non-biological processes, animals (and macroalgae) became a dominant source of calcareous material in Ordovician deposits.
Climate and sea level
Ordovician
The Early Ordovician climate was very hot, with intense greenhouse conditions and sea surface temperatures comparable to those during the Early Eocene Climatic Optimum. Carbon dioxide levels were very high at the Ordovician period's beginning. By the late Early Ordovician, the Earth cooled, giving way to a more temperate climate in the Middle Ordovician, with the Earth likely entering the Early Palaeozoic Ice Age during the Sandbian, and possibly as early as the Darriwilian or even the Floian. The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time. Evidence suggests that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe. The early Katian also witnessed yet another humidification event. Further cooling during the Hirnantian, at the end of the Ordovician, led to the Late Ordovician glaciation.
The Ordovician saw the highest sea levels of the Paleozoic, and the low relief of the continents led to many shelf deposits being formed under hundreds of metres of water. The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period. Locally, some regressions occurred, but the sea level rise continued in the beginning of the Late Ordovician. Sea levels fell steadily due to the cooling temperatures for about 3 million years leading up to the Hirnantian glaciation. During this icy stage, the sea level has risen and dropped somewhat. Despite much study, the details remain unresolved. In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation, but sedimentary evidence of glaciation is lacking until the end of the period. There is evidence of glaciers during the Hirnantian on the land we now know as Africa and South America, which were near the South Pole at the time, facilitating the formation of the ice caps of the Hirnantian glaciation.
As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. The Panthalassic Ocean covered much of the Northern Hemisphere, and other minor oceans included Proto-Tethys, Paleo-Tethys, Khanty Ocean, which was closed off by the Late Ordovician, Iapetus Ocean, and the new Rheic Ocean.
Life
Ordovician
A diorama depicting Ordovician flora and fauna
For most of the Late Ordovician life continued to flourish, but at and near the end of the period there were mass-extinction events that seriously affected conodonts and planktonic forms like graptolites. The trilobites Agnostida and Ptychopariida completely died out, and the Asaphida were much reduced. Brachiopods, bryozoans and echinoderms were also heavily affected, and the endocerid cephalopods died out completely, except for possible rare Silurian forms. The Ordovician–Silurian extinction events may have been caused by an ice age that occurred at the end of the Ordovician Period, due to the expansion of the first terrestrial plants, as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.
Fauna
Endoceras, one of the largest predators of the Ordovician
Fossiliferous limestone slab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.
The trilobite Isotelus from Wisconsin
On the whole, the fauna that emerged in the Ordovician were the template for the remainder of the Palaeozoic. The fauna was dominated by tiered communities of suspension feeders, mainly with short food chains. The ecological system reached a new grade of complexity far beyond that of the Cambrian fauna, which has persisted until the present day. Though less famous than the Cambrian explosion, the Ordovician radiation (also known as the Great Ordovician Biodiversification Event) was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna. Several animals also went through a miniaturization process, becoming much smaller than their Cambrian counterparts. Another change in the fauna was the strong increase in filter-feeding organisms. The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those that dominated the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids. Articulate brachiopods, in particular, largely replaced trilobites in shelf communities. Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.
Aegirocassis, a large filter-feeding hurdiid radiodont from Morocco
Ordovician geography had its effect on the diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism. The widely separated continents of Laurentia and Baltica, then positioned close to the tropics and boasting many shallow seas rich in life, developed distinct trilobite faunas from the trilobite fauna of Gondwana, and Gondwana developed distinct fauna in its tropical and temperature zones. The Tien Shan terrane maintained a biogeographic affinity with Gondwana, and the Alborz margin of Gondwana was linked biogeographically to South China. Southeast Asia's fauna also maintained strong affinities to Gondwana's. North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana. A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones. However, tropical articulate brachiopods had a more cosmopolitan distribution, with less diversity on different continents. During the Middle Ordovician, beta diversity began a significant decline as marine taxa began to disperse widely across space. Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean, though they were still distinguishable into the late Ordovician.
Pentecopterus, an early eurypterid, and found in Iowa
Trilobites in particular were rich and diverse, and experienced rapid diversification in many regions. Trilobites in the Ordovician were very different from their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitive eurypterids and nautiloids while other trilobites such as Aeglina prisca evolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as the trinucleids developed a broad pitted margin around their head shields. Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely. Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician. Although solitary corals date back to at least the Cambrian, reef-forming corals appeared in the early Ordovician, including the earliest known octocorals, corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals. Brachiopods surged in diversity, adapting to almost every type of marine environment. Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation. Molluscs, which appeared during the Cambrian or even the Ediacaran, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods. Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments. Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans. This includes the distinctive Nemagraptus gracilis graptolite fauna, which was distributed widely during peak sea levels in the Sandbian. Some new cystoids and crinoids appeared. It was long thought that the first true vertebrates (fish — Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian. The first gnathostome (jawed fish) may have appeared in the Late Ordovician epoch. Chitinozoans, which first appeared late in the Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread. Several groups of endobiotic symbionts appeared in the Ordovician.
In the Early Ordovician, trilobites were joined by many new types of organisms, including tabulate corals, strophomenid, rhynchonellid, and many new orthid brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ("brittle stars") and the first sea stars. Nevertheless, the arthropods remained abundant; all the Late Cambrian orders continued, and were joined by the new group Phacopida. The first evidence of land plants also appeared (see evolutionary history of life).
In the Middle Ordovician, the trilobite-dominated Early Ordovician communities were replaced by generally more mixed ecosystems, in which brachiopods, bryozoans, molluscs, cornulitids, tentaculitids and echinoderms all flourished, tabulate corals diversified and the first rugose corals appeared. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. One of the earliest known armoured agnathan ("ostracoderm") vertebrates, Arandaspis, dates from the Middle Ordovician. During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution. It is marked by a sudden abundance of hard substrate trace fossils such as Trypanites, Palaeosabella, Petroxestes and Osprioneides. Bioerosion became an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensive carbonate hardgrounds that appear in abundance at this time.
• Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky with the cyclostome bryozoan Corynotrypa in the background • Middle Ordovician fossiliferous shales and limestones at Fossil Mountain, west-central Utah • Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana • Outcrop of Upper Ordovician limestone and minor shale, central Tennessee • Trypanites borings in an Ordovician hardground, southeastern Indiana • Petroxestes borings in an Ordovician hardground, southern Ohio • Outcrop of Ordovician kukersite oil shale, northern Estonia • Bryozoan fossils in Ordovician kukersite oil shale, northern Estonia • Brachiopods and bryozoans in an Ordovician limestone, southern Minnesota • Vinlandostrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0 mm) • The Ordovician cystoid Echinosphaerites (an extinct echinoderm) from northeastern Estonia; approximately 5 cm in diameter • Prasopora, a trepostome bryozoan from the Ordovician of Iowa • An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan • The heliolitid coral Protaraea richmondensis encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana • Zygospira modesta, atrypid brachiopods, preserved in their original positions on a trepostome bryozoan from the Cincinnatian (Upper Ordovician) of southeastern Indiana • Graptolites (Amplexograptus) from the Ordovician near Caney Springs, Tennessee
Flora
Green algae were common in the Late Cambrian (perhaps earlier) and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing as tiny non-vascular forms resembling liverworts, in the middle to late Ordovician. Fossil spores found in Ordovician sedimentary rock are typical of bryophytes.
Colonization of land would have been limited to shorelines
Among the first land fungi may have been arbuscular mycorrhiza fungi (Glomerales), playing a crucial role in facilitating the colonization of land by plants through mycorrhizal symbiosis, which makes mineral nutrients available to plant cells; such fossilized fungal hyphae and spores from the Ordovician of Wisconsin have been found with an age of about 460 million years ago, a time when the land flora most likely only consisted of plants similar to non-vascular bryophytes.
Microbiota
Though stromatolites had declined from their peak in the Proterozoic, they continued to exist in localised settings.
End of the period
Ordovician
The Anji Biota (Wenchang Formation, Zhejiang Province, China) preserves abundant and diverse glass sponges and graptolites as well as rare examples of other marine animals (such as the eurypterid Archopterus) living at a depth of several hundred metres. It is dated to just after the Hirnantian mass extinction at the end of the Ordovician period.
The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. The only larger one was the Permian–Triassic extinction event.
The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the following Silurian Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; brachiopods and bryozoans were greatly reduced, along with many trilobite, conodont and graptolite families.
The most commonly accepted theory is that these events were triggered by the onset of cold conditions in the late Katian, followed by an ice age, in the Hirnantian faunal stage, that ended the long, stable greenhouse conditions typical of the Ordovician.
The ice age was possibly not long-lasting. Oxygen isotopes in fossil brachiopods show its duration may have been only 0.5 to 1.5 million years. Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.
The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm). The dip may have been caused by a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode. Another possibility is that bryophytes and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down CO2 levels. The drop in CO2 selectively affected the shallow seas where most organisms lived. It has also been suggested that shielding of the sun's rays from the proposed Ordovician ring system, which also caused the Ordovician meteor event, may have also led to the glaciation. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.
As glaciers grew, the sea level dropped, and the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches. When they returned, they carried diminished founder populations that lacked many whole families of organisms. They then withdrew again with the next pulse of glaciation, eliminating biological diversity with each change. Species limited to a single epicontinental sea on a given landmass were severely affected. Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.
Those species able to adapt to the changing conditions survived to fill the ecological niches left by the extinctions. For example, there is evidence the oceans became more deeply oxygenated during the glaciation, allowing unusual benthic organisms (Hirnantian fauna) to colonize the depths. These organisms were cosmopolitan in distribution and present at most latitudes.
At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders. Recovery was characterized by an unusual number of "Lazarus taxa", disappearing during the extinction and reappearing well into the Silurian, which suggests that the taxa survived in small numbers in refugia.
An alternate extinction hypothesis suggested that a ten-second gamma-ray burst could have destroyed the ozone layer and exposed terrestrial and marine surface-dwelling life to deadly ultraviolet radiation and initiated global cooling.
Recent work considering the sequence stratigraphy of the Late Ordovician argues that the mass extinction was a single protracted episode lasting several hundred thousand years, with abrupt changes in water depth and sedimentation rate producing two pulses of last occurrences of species.