Period· The columns of time

Ordovician

485 Ma → 444 Ma

The Cambrian settled the body plans; the Ordovician filled the sea with them, then the ice carried much of that life away.

What the evidence shows

Marine families multiply several times over, reef building passes from microbes to animals, and plankton with its own predators becomes a world in itself: taken together, these rises are called the Great Ordovician Biodiversification Event, an increase in variety far larger than the Cambrian explosion and far less famous. Land changes too. Around 470 million years ago, plants small, flat and rootless leave the oldest known trace of vegetation out of water, spores scattered by the million through the rock and already varied enough to imply a group older than its own record. Then it ends in ice. Tropical seas that had held at 32 to 37 degrees drop about five, ice sheets spread over the south pole, sea level falls by roughly a hundred metres, and the shelf seas where life is thickest drain away: the first of the five great extinctions, and the only one that comes with an ice age.

Bounds and conventions

Its bounds are golden spikes and firm. What happens inside is less so: the size of the cooling is reconstructed from the isotopic composition of fossil shells, a record in which ocean temperature and ice volume are hard to tell apart, and the causes of the extinction are still listed in the plural. The asteroid breakup that showered the mid-Ordovician with meteorites was long linked to the cooling, and a refined timescale has since separated the two.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Servais & Harper (2018), Lethaia · The Great Ordovician Biodiversification Event: definition, concept and duration
3. Rubinstein et al. (2010), New Phytologist · Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana)
4. Finnegan et al. (2011), Science · The magnitude and duration of Late Ordovician-Early Silurian glaciation
5. Harper (2024), National Science Review · Late Ordovician Mass Extinction: Earth, fire and ice
6. Schmitz et al. (2019), Science Advances · An extraterrestrial trigger for the mid-Ordovician ice age: dust from the breakup of the L-chondrite parent body