Period· The columns of time

Ordovician

487 Ma → 443 Ma

The Ordovician, from 485 to 444 million years ago, multiplied marine life, saw plants reach the continents and closed with the first of the five great extinctions.

Period of major marine diversification, in which plants reach the continents, closed by the first of the five great extinctions.

What the evidence shows

Marine families become several times more numerous, reef building passes from microbes to animals, and the plankton, with predators of its own, constitutes a distinct ecosystem: the whole is called the Great Ordovician Biodiversification Event, an increase in variety more extensive than the Cambrian explosion. The continents change as well. About 470 million years ago, small, flat, rootless plants leave the oldest known trace of vegetation out of water, as cryptospores scattered in millions through the rock and already varied enough to indicate a group older than its own record. The period ends in glaciation: tropical seas standing between 32 and 37 degrees lose about five, ice sheets spread over the south pole, sea level falls by about a hundred metres, and the shelf seas in which life is densest drain away.

Bounds and conventions

Its limits are golden spikes and are stable. Its contents less so: the magnitude of the cooling is reconstructed from the isotopic composition of fossil shells, a record in which ocean temperature and ice volume are hard to separate, and the causes of the extinction are still cited in the plural. The asteroid break-up that peppered the mid-Ordovician with meteorites was long connected with the cooling, and a refined chronology has since separated the two.