420 Million B.C.T. - The Lau Event Occurred

Around 420 Million B.C.T., the Lau event -- the last mass extinction of the Silurian period, occurred.

There were three minor extinction events that occurred during the Silurian period.  The extinction events were the Ireviken (433 Million B.C.T.), the Mulde (423 Million B.C.T.) and the Lau (420 Million B.C.T.

The Ireviken event was the first minor extinction event that occurred during the mid Silurian period around 433.4 ± 2.3 million years ago. The event is best recorded at Ireviken, Gotland (Sweden), where over 50% of trilobite species went extinct; 80% of the global conodont species also became extinct in this interval.

The event lasted around 200,000 years.  It comprises eight extinction "datum points"—the first four being regularly spaced, every 30,797 years, and linked to the Milankovic obliquity cycle. The fifth and sixth probably reflect maxima in the precessional cycles, with periods of around 16.5 and 19 thousand years. The final two data are much further spaced, so harder to link with Milankovic changes.

The mechanism responsible for the Ireviken 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.


The Mulde event was the second minor mass extinction event of the Silurian Period.  It coincided with a global drop in sea level, and is closely followed by an excursion in geochemical isotopes.


The Lau event was the last of the three relatively minor mass extinctions during the Silurian period, having a major effect on the conodont fauna (but barely scathing the graptolites). It coincided with a global low point in sea level, was closely followed by an excursion in geochemical isotopes in the ensuing late Ludfordian faunal stage and a change in depositional regime.


The Lau event started about 420 million years ago. Its strata are best exposed in Gotland, Sweden, taking its name from the parish of Lau. Its base is set at the first extinction datum, in the Eke beds, and despite a scarcity of data, it is apparent that most major groups suffered an increase in extinction rate during the event; major changes are observed worldwide at correlated rocks, with a "crisis" observed in populations of conodonts and graptolites. More precisely, conodonts suffered in the Lau event, and graptolites in the subsequent isotopic excursion. Local extinctions may have played a role in many places, especially the increasingly enclosed Welsh basin.  The event's relatively high severity rating of 6.2 does not change the fact that many life-forms became re-established shortly after the event, presumably surviving in refuge or in environments that have not been preserved in the geological record. Although life persisted after the event, community structures were permanently altered and many lifeforms failed to regain the niches they had existed in prior to the event.