Teeming seas, then a great freeze
Ordovician
Paleozoic Era · 485.4 – 443.8 million years ago
The Ordovician Period ran from about 485.4 to 443.8 million years ago, a span of roughly 42 million years. It saw one of the greatest surges in the diversity of marine life in Earth's history, and ended with the first of the 'Big Five' mass extinctions.
Ordovician seas were the highest of the past half-billion years, spreading warm shallow water across the continents. Yet by its close, a vast ice sheet had grown over what is now the Sahara, and the resulting crisis wiped out most marine species.
Key facts
- Began
- 485.4 MaMillion years ago
- Ended
- 443.8 MaMillion years ago
- Duration
- 41.6 million years
- Era
- Paleozoic“Ancient life”
- Global temperature
- ~19°CRoughly 14–21°C · today 14.5°C
- Plants on land
- Mosses & liverworts
- Atmospheric CO₂
- High, perhaps several thousand ppm, falling late
- Oxygen
- Rising, but likely below modern levels
- Sea level
- Among the highest of the Phanerozoic
- Continents
- Gondwana over the South Pole; Laurentia on the equator
- Signature life
- Trilobites, graptolites, nautiloids, brachiopods
- Began with
- Recovery and diversification after the Cambrian
- Ended with
- Hirnantian glaciation and the end-Ordovician extinction
Ages follow the International Chronostratigraphic Chart (ICS). Global temperatures are rough reconstructions after Scotese et al. (2021); ancient CO₂, oxygen and sea-level values carry large uncertainties.
Epochs of the Ordovician
- Early Ordovician485.4 Ma – 470 Ma
- Middle Ordovician470 Ma – 458.4 Ma
- Late Ordovician458.4 Ma – 443.8 Ma
Continents and oceans
Gondwana dominated the southern hemisphere and drifted over the South Pole during the period, with what is now North Africa lying close to the pole by the Late Ordovician. Laurentia sat astride the equator, while Siberia and Baltica lay in lower latitudes to the east and south.
Early in the period the microcontinent Avalonia, including parts of today's England, Wales, Newfoundland and New England, broke away from Gondwana. Its departure opened the Rheic Ocean behind it as it moved north, while the Iapetus Ocean ahead of it began to narrow.
Volcanic island arcs collided with the edge of Laurentia, raising the Taconic mountains in what is now eastern North America. Intense volcanism spread ash layers across huge areas, some of which can still be traced over hundreds of kilometres.
Climate and atmosphere
The Early Ordovician appears to have been very hot, but oxygen isotopes from fossil conodonts suggest the oceans cooled steadily through the Middle Ordovician. Many scientists think this cooling helped trigger the explosion of marine diversity by creating more favourable conditions in the tropics.
By the end of the period, carbon dioxide had fallen enough for ice to build up over the polar part of Gondwana during the Hirnantian Age. Possible causes include weathering of fresh volcanic and mountain rocks and, some researchers argue, the spread of early land plants, but the balance of causes is still debated.
Life in the Ordovician
During the Great Ordovician Biodiversification Event (GOBE), the number of marine families roughly tripled. Rather than inventing new body plans, existing groups branched into many more species and ecological roles, building ecosystems that would dominate the Paleozoic seas.
In the seas
Brachiopods, crinoids, bryozoans and trilobites flourished on the seafloor, while graptolites drifted in the plankton. Straight-shelled nautiloids such as Endoceras were top predators, some reaching several metres in length.
Reefs
New reef builders appeared, including tabulate and rugose corals, stromatoporoid sponges and bryozoans. These reefs were more complex than anything before them and resemble later Paleozoic reefs.
Early vertebrates
Jawless, armoured fish such as Arandaspis, Astraspis and Sacabambaspis lived in coastal waters. They had no paired fins or jaws, but their bony plates are among the earliest well-preserved vertebrate fossils.
On land
Microscopic spores from around 470 million years ago show that simple plants, probably similar to liverworts, had begun to colonise damp ground. They left no large fossils, but mark the start of life's move onto land.
Timeline of key events
- 485.4 Ma
Ordovician begins
The period starts with the appearance of a key conodont species at Green Point, Newfoundland.
- 478 Ma
Fezouata ecosystems thrive
Soft-bodied animals, including giant filter-feeding radiodonts, are preserved in what is now Morocco.
- 470 Ma
First land plant spores
Spores of simple, liverwort-like plants show that plants had begun to colonise land.
- 466 Ma
Asteroid break-up showers Earth
The break-up of a large asteroid sends a lasting surge of meteorites to Earth, preserved in Swedish limestones.
- 460 Ma
Biodiversification peaks
The Great Ordovician Biodiversification Event reaches its height as marine diversity soars.
- 445 Ma
Hirnantian glaciation begins
Ice sheets spread over Gondwana and the first pulse of the end-Ordovician extinction strikes.
- 443.8 Ma
Second extinction pulse
As the ice melts, low-oxygen waters spread, and the period closes with a second wave of extinctions.
How the Ordovician ended
The end-Ordovician mass extinction struck in two main pulses between about 445 and 443 million years ago. The first came as ice spread across Gondwana, cooling the oceans and dropping sea level by tens of metres, which drained shallow continental seas. The second followed when the ice melted and low-oxygen water spread over the shelves.
In total an estimated 85% of marine species disappeared, making this the second-largest of the Big Five extinctions by species lost. Trilobites, brachiopods, graptolites and conodonts were all hit hard, but the ecosystems that survived rebuilt along broadly similar lines.
Where to see Ordovician fossils
The area around Cincinnati, Ohio, and neighbouring Kentucky and Indiana is famous for limestones packed with brachiopods, bryozoans and trilobites. In Morocco, the Fezouata Formation preserves soft-bodied Early Ordovician animals, while the Soom Shale of South Africa records life during the end-Ordovician glaciation.
Beecher's Trilobite Bed in New York State contains trilobites preserved in pyrite with their legs intact. The region around St Petersburg in Russia yields superb trilobites, and Ordovician rocks are also well exposed in Wales and in Sweden, including limestone quarries at Kinnekulle.
Surprising facts about the Ordovician
- The Sahara was once glaciated: rocks in Algeria and Libya preserve scratched surfaces and deposits left by Late Ordovician ice sheets, when North Africa lay near the South Pole.
- Limestones in southern Sweden contain dozens of fossil meteorites, evidence that meteorite falls were far more frequent after an asteroid broke up about 466 million years ago.
- One of the largest known arthropods of its time, the radiodont Aegirocassis from Morocco, was around 2 metres long and filtered plankton like a whale.
- The name Ordovician honours the Ordovices, an ancient tribe of north Wales. Charles Lapworth proposed it in 1879 to settle a bitter dispute between Sedgwick and Murchison over overlapping rock units.
- Sea level was so high that shallow seas covered much of the interior of North America, and some estimates put it more than 100 metres above today's level.
- Graptolites evolved so quickly and spread so widely that geologists use them to date Ordovician rocks with precision, sometimes to within a million years or so.
Where it sits in deep time
The globe beside this article shows Earth 455 million years ago, reconstructed from the PALEOMAP paleo-elevation models. Step to another period and watch the continents drift into place.