The Ireviken event
An anoxic pulse kills half the world's trilobite species and four-fifths of its conodonts.
- When
- Known to
- ± 1.2 million years
- Era
- Silurian, Paleozoic
At the Llandovery–Wenlock boundary, about 432.9 million years ago, oxygen-poor water spreads across the shelves. Over 50% of trilobite species and around 80% of conodont species are lost, along with much of the graptolite fauna. It is the first of three Silurian crises severe enough to reset marine communities without qualifying as a mass extinction — a reminder that the 'big five' are the peaks of a continuous distribution.