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Great Ordovician Biodiversification Event
c. 485–445 million years ago· Event· Animals
Well establishedStrong fossil, genetic or geological evidence.
A roughly 40-million-year surge in which the number of marine animal species and families multiplied several times over.
See it on the Life map →What happened
The Cambrian explosion produced new body plans; the Ordovician filled the seas with species. Marine families roughly tripled and genera rose about fourfold. Reefs built by sponges, corals and bryozoans spread. Brachiopods, crinoids ('sea lilies'), trilobites, snails and shelled nautiloids, relatives of squid, thrived, and floating colonies called graptolites boomed. High sea levels flooded continents, creating wide, warm, shallow seas.
Why it matters
It built the shelled seafloor communities that dominated the rest of the Paleozoic, and it set the baseline the first great mass extinction struck.
Think of it like
Like a development cycle: the Cambrian laid out the property types; the Ordovician built out the lots.
The evidence (fossils, genes, rocks)
Global fossil databases tallying species and genera through dated rock layers worldwide, building on Jack Sepkoski's counts of marine families from the 1980s.
Still debated
What drove it is debated: cooling seas, more oxygen, a plankton boom, scattered continents, even debris from an asteroid breakup about 466 million years ago.
Inside this (1)
Connections (3)
▸Great Ordovician Biodiversification Event was cut back by Late Ordovician mass extinctionWell established
The connection
The Ordovician boom filled shallow tropical seas with brachiopods, trilobites, reef builders and plankton. When ice spread over Gondwana, those seas drained and cooled, and specialists of warm shallow water died in large numbers. About 85% of marine species vanished, though many families survived to rebuild.
When and how
Oceans worldwide, about 445–443 million years ago, in the Hirnantian, the last stage of the Ordovician.
The evidence
Fossil counts through dated rock sequences show diversity rising for some 40 million years, then falling sharply in two steps matched by glacial and isotope evidence.
Debates and open questions
Why it was so severe in species lost yet changed ecosystem structure less than other mass extinctions did.
▸Great Ordovician Biodiversification Event diversified TrilobitesWell established
The connection
Trilobites appeared in the Cambrian, but in the Ordovician new trilobite groups spread into new habitats and lifestyles: open-water swimmers, deep-water forms, burrowers, and species with huge eyes or long spines. Meanwhile older Cambrian-style groups declined, before heavy losses in the Late Ordovician extinction.
When and how
Shallow and deep seas worldwide, about 485–445 million years ago, around the continents of Laurentia, Baltica and Gondwana.
The evidence
Fossil databases record many new trilobite families and genera in Ordovician rocks, with new lifestyles visible in their anatomy.
Debates and open questions
Why some trilobite groups thrived in the radiation while older ones faded: competition, habitat change or chance.
▸Cambrian explosion was followed by Great Ordovician Biodiversification EventWell established
The connection
The Cambrian explosion created most major animal body plans. The Ordovician radiation did something different: it multiplied species and families within those body plans, filling new niches such as reefs, open water and deeper seafloor. One set up the basic designs; the next filled the world with variations on them.
When and how
Seas worldwide, from about 539 to 445 million years ago, with a quieter late Cambrian interval in between.
The evidence
Fossil counts through dated rock layers show body plans appearing first and species counts climbing later; global databases confirm the pattern.
Debates and open questions
Some researchers argue the two were one continuous radiation, and that the late Cambrian lull reflects a poor fossil record.