It occurred at the Llandovery/Wenlock boundary (mid Silurian, 432.9 ± 1.2 million years ago).
[5][6][2] It comprises eight extinction "datum points"—the first four being regularly spaced, every 30,797 years, and linked to the Milankovic obliquity cycle.
[4] The fifth and sixth probably reflect maxima in the precessional cycles, with periods of around 16.5 and 19 ka.
[4] The mechanism responsible for the event originated in the deep oceans, and made its way into the shallower shelf seas.
Correspondingly, shallow-water reefs were barely affected, while pelagic and hemipelagic organisms such as the graptolites, conodonts and trilobites were hit hardest.