Earth's first great extinction came as ice, then as suffocating warmth
Roughly 445 million years ago, almost all complex life lived in the sea, and nearly 85 percent of marine species disappeared. The killer struck twice: first a sudden ice age over Gondwana that drained shallow seas, then a rapid thaw that stripped the oceans of oxygen and filled them with toxic sulfide.
The Late Ordovician mass extinction is the earliest of the Big Five and, by the share of genera lost, usually ranked second only to the end-Permian catastrophe. Between 49 and 60 percent of marine genera vanished, around 100 families in all. It came right after the Great Ordovician Biodiversification Event, one of the biggest bursts of new life in Earth's history. On land there was almost nothing yet, only rare spores from tiny early plants.
The opening blow landed where the Katian stage gives way to the Hirnantian, when glaciers spread abruptly across Gondwana and flipped the planet from greenhouse to icehouse. Cooling and falling sea levels destroyed habitats on the continental shelves, and species with narrow temperature ranges suffered most. The Foliomena fauna, thin-shelled animals adapted to deep, low-oxygen water, disappeared entirely even though it had long resisted background extinction. A new cold-water community, the Hirnantia fauna, spread into lower latitudes in response.
That newcomer did not last. In the second pulse, later in the Hirnantian, the ice retreated quickly and warmth returned, bringing worldwide anoxia and euxinia that lingered into the Silurian. The Hirnantia fauna was replaced by warm-water assemblages. Some researchers argue for a third pulse in the early Silurian, while others treat it as a drawn-out tail of the second.
Brachiopods and bryozoans each lost about a third of their families, and trilobites, conodonts and graptolites were badly hit. Yet the long-term ecological damage was modest compared with later extinctions, and recovery came much faster than after the Permian crisis. Silurian seas ended up filled with some of the most widespread, cosmopolitan faunas of the entire Phanerozoic, replacing the highly local communities of the Ordovician.
Source: Late Ordovician mass extinction
