LIFE × EVOLUTION

The Ordovician Fills the Sea

~485 to 443 million years ago

Phanerozoic · Paleozoic · Ordovician

The Cambrian settled the body plans; the Ordovician filled the sea with them. Families multiplied several times over, reef building passed from microbes to animals, and plankton, with predators of its own, became a world in itself, layered above the seafloor instead of spread across it.

Why it matters

The modern ocean, with its plankton, its open water and its animal reefs, dates from here rather than from the Cambrian. It was not built in a stroke: the diversification runs the length of the period, group by group and region by region, which is why specialists disagree over whether one name should cover it at all.

Dating & uncertainty

Whether this is one event or many remains open. Diversity rises at different times from one group to another and from one region to another, and one school holds that calling it a single global event is an artefact of pooling unevenly sampled databases.

Sources

1. Servais et al. (2010), Palaeogeography, Palaeoclimatology, Palaeoecology · The Great Ordovician Biodiversification Event: the palaeoecological dimension
2. Servais & Harper (2018), Lethaia · The Great Ordovician Biodiversification Event: definition, concept and duration
3. Servais et al. (2021), Paleontological Research · The Great Ordovician Biodiversification Event is not a single event

Earth history as one day

21:26:102 h 34 min before midnight

if Earth’s 4.54 billion years were compressed into a single day