EXTINCTIONS × CLIMATE

The First Mass Extinction

~445 million years ago

Phanerozoic · Paleozoic · Ordovician

The first of the five great crises, and the only one caused by cold. Tropical seas that had held at 32 to 37 degrees dropped about five, ice sheets grew to a volume equal to or greater than the last Ice Age, sea level fell some hundred metres, and the shallow shelf seas where almost everything lived drained away. Toxic water moved up onto what was left of the shelves. It killed in two pulses.

Why it matters

Enormous numbers of species died, but few whole branches of the tree were lost and the structure of the ecosystem came through: a massive taxonomic extinction, and a comparatively mild ecological one. What it cost was time. Diversity did not return to its previous level for at least thirty-five million years.

Dating & uncertainty

Two pulses bracket the Hirnantian glaciation. Their relative severity is disputed, as is the trigger: cooling, ocean chemistry and volcanic mercury spikes each have their advocates.

Sources

1. Finnegan et al. (2011), Science · The magnitude and duration of Late Ordovician-Early Silurian glaciation
2. Rasmussen et al. (2019), PNAS · Cascading trend of Early Paleozoic marine radiations paused by Late Ordovician extinctions
3. Harper et al. (2014), Gondwana Research · End Ordovician extinctions: a coincidence of causes
4. Harper (2024), National Science Review · Late Ordovician Mass Extinction: Earth, fire and ice

Earth history as one day

21:38:512 h 21 min before midnight

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