Beginning of Silurian
The end of the Ordovician–Silurian extinction event occurred when melting glaciers caused the sea level to rise and eventually stabilize. Biodiversity, with the sustained re-flooding of continental shelves at the onset of the Silurian, rebounded within the surviving orders.
Following the major loss of diversity as the end-Ordovician, Silurian communities were initially less complex and broader niched. Highly
endemic faunas, which characterized the Late Ordovician, were replaced by faunas that were amongst the most cosmopolitan in the Phanerozoic, biogeographic patterns that persisted throughout most of the Silurian.
These end Ordovician–Silurian events had nothing like the long-term impact of the Permian–Triassic and Cretaceous-Paleogene extinction events. Nevertheless, a large number of taxa disappeared from the Earth over a short time interval, eliminating and changing diversity.
The epoch was named after
Hartfell Shale (48 metres (157 ft) thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites. Above this is the 43 metres (141 ft) thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites.
The base was originally defined as the first appearance of the graptolite Akidograptus ascensus at Dob's Linn, but was later discovered to be imprecise.  It is currently placed between acritarch biozone 5 and last appearance of Pterospathodus amorphognathoides.
It has been recommended to place the GSSP at a slightly higher and correlatable level on the Ireviken datum 2, which coincides approximately with the base of the murchisoni Graptolite Biozone.
In North America a different suite of regional stages is sometimes used:
Spores and plant microfossils have been found in China and Pennsylvania. There was some movement to the land during the Llandovery but the earliest known vascular plants (Cooksonia) have only been found in rocks of the middle Silurian.
Parioscorpio venator was at first described as the earliest fossil land animal in 2020. It was originally described as the oldest known scorpion (437 million years old), but was later re-described as an enigmatic, marine arthropod.
bioherms. Rising water temperatures in the Devonian might have led to bleaching of these corals.
The Ireviken event was the first of three relatively minor extinction events (the Ireviken, Mulde, and Lau events) during the Silurian Period. The Ireviken overlapped the Llandovery/Wenlock boundary. The event is best recorded at Ireviken, Gotland.
Anatomy of the event
It comprises eight extinction "datum points"—the first four being regularly spaced, every 31,000 years, and linked to the
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. 50% of trilobite species and 80% of the global conodont species become extinct in this interval.