Ordovician Period

The Ordovician Period lasted from 486.85 ± 1.5 to 443.1 ± 0.9 million years ago. It was the second period of the Palaeozoic Era, between the Cambrian and Silurian. Almost all conspicuous life still lived in water, but marine communities became far more diverse and structurally complex. Brachiopods, bryozoans, echinoderms and reef builders spread across the bottom, graptolites and cephalopods occupied the water column, and small jawless vertebrates were present.

The Ordovician cannot be reduced to one simple “flowering of marine life”. Its beginning retained unusual animals with a Cambrian character. The long Great Ordovician Biodiversification Event then unfolded in several regional waves. At the very end of the period, ice grew across Gondwana, sea level fell, and the subsequent reorganisation of the oceans produced one of the five largest mass extinctions of the Phanerozoic.

The Ordovician ocean was rich but not modern. There were no ammonites, bony fishes, marine reptiles or dinosaurs. On land there were no trees, grasses or animals securely documented by complete terrestrial skeletons.

Why the period is called Ordovician

British geologist Charles Lapworth proposed the name in 1879. It comes from the Ordovices, a Celtic people who lived in what is now Wales. The new system helped resolve a long dispute: Adam Sedgwick assigned part of the Welsh sequence to the Upper Cambrian, while Roderick Murchison included an overlapping interval in the Lower Silurian.

Lapworth used fossil succession to show that an independent package of rocks and faunas lay between the two systems. International acceptance was gradual, but the Ordovician eventually became a formal system. It is therefore not an arbitrary transition cut from the Cambrian and Silurian. It has its own stratigraphic content, reference boundaries and global subdivisions.

Where the Ordovician begins

The lower boundary also marks the start of the Tremadocian Stage. Its Global Boundary Stratotype Section and Point lies at Green Point on the western coast of Newfoundland, Canada. The formal level is in bed 23, 101.8 metres above the base of the measured section. The primary biostratigraphic marker is the first appearance of the conodont Iapetognathus fluctivagus.

This physical definition is more important than a rounded numerical date. The estimate of 486.85 ± 1.5 million years may be recalibrated, but the boundary remains tied to the agreed layer. Rocks below belong to the Cambrian Period; the Ordovician record begins above it.

Three epochs and seven stages

International stages do not perfectly match older regional schemes used in Britain, North America, Baltica and China. “Upper Ordovician” properly describes rocks, while “Late Ordovician” describes time, although popular accounts often mix the terms. The boundary with the Silurian lies at 443.1 ± 0.9 million years ago.

Continents, shelves and the Iapetus Ocean

Gondwana was the largest landmass and moved through southern latitudes towards the pole. Laurentia lay near the equator, separated from Baltica and Avalonia by the Iapetus Ocean. Siberia and several smaller blocks remained distinct. High sea level flooded broad continental margins and created extensive epicontinental seas.

Warm shallow waters provided many partly isolated arenas for evolution. Basins differed in temperature, salinity, depth, substrate and connection with the open ocean. Movement of Baltica and Avalonia towards Laurentia narrowed Iapetus. Along Laurentia, subduction and collision with an island arc produced the Taconic orogeny.

Modern national borders cannot simply be placed on an Ordovician map. Rocks now exposed in Wales, Newfoundland, Estonia, China and Morocco formed on different palaeocontinents and at different latitudes. Their present separation or proximity arose hundreds of millions of years later.

Climate from greenhouse conditions to ice sheets

For much of the period, climate was relatively warm and sea level high. “Greenhouse” does not mean one temperature at every latitude for 44 million years. Isotope evidence indicates long-term cooling with shorter fluctuations superimposed. By the Middle Ordovician, tropical seas may have cooled from the extreme warmth of the earliest Palaeozoic, expanding habitable depth and latitude ranges without acting as the sole cause of diversification.

Late in the Katian and during the Hirnantian, ice sheets grew rapidly on Gondwana. Ice locked up water and the sea withdrew from enormous shelves. Shallow-water organisms faced more than falling temperature: the area of habitat contracted, currents and salinity changed, and oxygen distribution shifted.

Fezouata: a window into the Early Ordovician

The Fezouata Shale of Morocco preserves an Early Ordovician biota about 478 million years old. Ordinary fossil records favour shells and armour, but fine sediment here retained outlines and mineral films of soft parts. Fezouata therefore bridges the famous Cambrian soft-bodied faunas and more familiar later Palaeozoic communities.

Radiodonts, marrellomorphs, nektaspids, worms, trilobites and echinoderms occur in the assemblage. Some lineages resemble Cambrian forms, while others belong to groups that became more prominent later. The large Aegirocassis had filtering frontal appendages, demonstrating that radiodonts were not all active predators. Comparison with Anomalocaris shows the range of feeding strategies within the broader group.

Fezouata contradicts a sharp replacement of faunas exactly at the period boundary. Ecological reorganisation continued after the Cambrian radiation, and surviving Cambrian-style branches coexisted with new elements of Ordovician ecosystems.

The Great Ordovician Biodiversification Event

The Great Ordovician Biodiversification Event is sometimes presented as a second sudden explosion. It was better understood as a long, uneven process. Diversity began and accelerated at different times in different groups and seas. Numbers of marine genera and families rose strongly, and regional communities became more distinct.

Animals occupied more space above the bottom, penetrated sediment more deeply and formed more complicated food webs. Suspension feeders lived at several heights, predators and scavengers became more varied, and burrows altered sediment chemistry. Plankton also diversified, linking surface productivity with seafloor communities.

No single trigger explains all of this. Long-term cooling, high sea level, continental fragmentation, nutrient supply, oxygenation, ecological interactions and evolutionary innovations probably reinforced one another. An asteroid breakup has been invoked because extraterrestrial chromite becomes abundant in some Middle Ordovician sediments, but the timing does not make dust the sole global cause.

Diversity counts also depend on sampling. A richly collected limestone may yield many named genera, while a contemporaneous muddy basin preserves a different set of bodies. Taxonomic revision can combine several old names or separate one broad genus into several. Researchers therefore compare occurrence databases, ecological structure and phylogenetic patterns rather than reading one raw curve as a complete census of the oceans.

Trilobites and other arthropods

Trilobites remained important but were not one uniform ecological type. Broad asaphids moved over or partly within the substrate, while spiny and streamlined forms occupied other settings. Asaphus includes species with elevated eyes, and lines of Ampyx preserve unusually strong evidence for coordinated movement. Moulted exoskeletons complicate abundance counts because one animal could leave many separate pieces.

Other arthropods included ostracods, eurypterids and lingering radiodonts. Their mineralised parts preserve unevenly. Limbs, gills and digestive systems are usually inferred from rare exceptional fossils or relatives rather than from an average shell bed.

Brachiopods, bryozoans and echinoderms

Articulate brachiopods became major suspension feeders on many shelves. They may resemble bivalve molluscs, but their shells are arranged differently and the groups are unrelated. Bryozoans built branching or encrusting colonies of tiny zooids. Their skeletons created hard surfaces and filtered particles from moving water.

Echinoderms included crinoids, cystoids and many extinct experiments in body design. Stems and plates often fell apart after death, so a bed of ossicles need not represent a living meadow buried intact. Articulated specimens are more informative about posture, while isolated elements can still document diversity and transport.

Reefs were built by changing partnerships

Early Ordovician reefs often combined microbial structures with sponges. Later reefs increasingly included stromatoporoids, tabulate corals, rugose corals, bryozoans and other encrusters. Modern scleractinian corals did not yet exist. Palaeozoic tabulates and rugosans should not be painted as exact copies of a present tropical reef.

A reef was a three-dimensional habitat, not background decoration. It provided firm attachment, shelter and zones with different current speed. Storms could break the framework and redistribute fragments, so fossil reefs record repeated construction, damage and cementation.

Cephalopods and open-water predators

Nautiloid cephalopods with straight, curved or coiled shells became widespread. Gas and liquid in shell chambers controlled buoyancy, while the soft body occupied the living chamber at the opening. Endocerids included large straight-shelled forms, but spectacular maximum sizes are often reconstructed from fragments and need caution.

A straight-shelled nautiloid did not necessarily swim heavy point first. Shell form, siphuncle position and the relation between centres of mass and buoyancy affected orientation. Some species may have moved horizontally, while others held a more vertical posture. Small arthropods and other animals were possible prey, but exact hunting sequences are rarely direct evidence. True ammonites had not yet evolved.

Graptolites were colonies of many zooids, and many lived in the water column. Rapidly changing, widely distributed species are useful for correlating shales. A branching carbon film can resemble a plant, but a graptolite was an animal colony.

Early vertebrates were jawless

Ordovician vertebrates are represented mainly by jawless forms. Sacabambaspis from Bolivia had a broad head shield, front-facing eyes, a row of gill plates and a flexible rear body. Astraspis from North America and Arandaspis from Australia are known from armour pieces and partial remains.

These animals lacked jaws and modern paired fins. Feeding is inferred from the mouth region, gill apparatus, wear and comparison with other jawless vertebrates. Some may have collected small particles near the bottom, but the popular label “vacuum-cleaner fish” is not a diagnosis. Microscopic scale-like remains have sometimes been interpreted as early jawed vertebrates, yet complete Ordovician bodies of such animals are not securely known.

Was there life on land?

Land was not sterile. Microorganisms occupied wet surfaces much earlier, and Ordovician cryptospores provide direct evidence for early terrestrial plants. Some spores occur in resistant pairs and tetrads within a common envelope. This connects their producers with early embryophytes and bryophyte-grade plants.

Spores do not reveal the whole plant. The producers were probably tiny thalli or low shoots in damp habitats, not forests or meadows. Roots, leaves and vascular trunks cannot be supplied confidently. Larger land plants with well-preserved anatomy appear later.

Possible tracks of terrestrial arthropods and molecular estimates are discussed, but their dating and interpretation vary. A trackway might record an aquatic animal making a brief excursion rather than a permanent land resident. The Ordovician therefore should not be portrayed with an established diverse terrestrial fauna.

The end-Ordovician mass extinction

The Late Ordovician extinction belongs among the five largest Phanerozoic crises. Precise percentages depend on the database and method, so a slogan such as “85 per cent of all life” implies false precision. Shelf-dwelling brachiopods, trilobites, bryozoans, graptolites, conodonts and reef communities suffered especially severe losses.

The first main pulse accompanied rapid cooling, Gondwanan glaciation and sea-level fall. Tropical shallow seas contracted and climate belts shifted. Organisms adapted to warm epicontinental seas lost habitat. The second main pulse came as the ice weakened and the sea returned. Warming did not mean immediate recovery: circulation changed and oxygen-poor, locally sulphidic water spread across shelves.

Cooling and warming therefore both accompany the extinction because they belong to different phases of one climate-ocean reorganisation. Changes in productivity, nutrient supply and seawater chemistry added stress. A nearby gamma-ray burst or one unrecorded impact lacks comparable direct geological support.

Survival and Silurian recovery

The crisis did not erase the Ordovician world. Many brachiopod, trilobite, mollusc, echinoderm and vertebrate branches crossed the boundary. Communities changed and recovery took time, but Silurian reefs and fish faunas were built from survivors rather than created on an empty planet.

A geological period ends where rocks can be correlated consistently, not where every organism disappears. The Ordovician enlarged the diversity and ecological depth of marine systems and passed complex food webs into the Silurian.

Important fossil sites

No section records the whole period. Fezouata preserves soft tissues but represents one early basin. Cincinnati limestones document later shallows while losing most soft-bodied organisms. The wider history comes from comparing continents, rocks and preservational filters.

What did not exist in the Ordovician

Dinosaurs appeared roughly 210 million years after the period ended. There were no marine reptiles, birds, mammals, flowering plants or forests. Nautiloids were neither ammonites nor squids inside cones, and the reefs were not built by modern corals.

Land plants left spores, but they cannot be expanded into fern thickets. Permanent complex land animals are not documented by complete bodies. Most recognisable Ordovician scenes belong underwater because the sea held nearly all diverse macroscopic communities.

How to read an Ordovician reconstruction

Shells, armour, mineralised plates, trails and spores are direct finds. Soft tissues survive at a few exceptional sites but elsewhere must be inferred from relatives and muscle attachments. Colour, group density and a particular interaction are commonly artistic hypotheses. A useful reconstruction checks that organisms overlap in age and place, avoids modern corals and fishes, respects measured size, and states where inference begins.