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 Llandovery in Wales. The GSSP for the Silurian is located in a section at Dob's Linn in an artificial excavation created just north of the Linn Branch Stream. Two lithological units occur near the boundary. The lower is the Hartfell Shale, consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites. Above this is the 43m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites. The base is identified by the appearance of the graptolites Parakidograptus acuminatus and Akidograptus ascensus at Dob's Linn.
In North America a different suite of regional stages is sometimes used:
Ontarian
Alexandrian
In Estonia the following suite of regional stages is used:
Adavere stage
Raikküla stage
Juuru stage
Palaeontology
Plants
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 have only been found in rocks of the middle Silurian.
Land animals
Parioscorpio venator was described as the earliest fossil land animal in 2020. It is an early scorpion with respiratory structures that indicate that it was able to live on land.
Reef expansion
systems covered a substantially greater percentage of seafloor than reefs today and they also grew at high latitudes. Possibly the evolution of photo symbionts started in the Llandovery epoch. Tabulate corals mostly developed as prominent 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 during the Silurian period. The Ireviken overlapped the Llandovery/Wenlock boundary. The event is best recorded at Ireviken, Gotland.
Anatomy of the event
The event lasted around 200,000 years, spanning the base of the Wenlock epoch. 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 ka. The final two data are much further spaced, so harder to link with Milankovic changes.
Casualties
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.
Geochemistry
Subsequent to the first extinctions, excursions in the δ13C and δ18O records are observed; δ13C rises from +1.4‰ to +4.5‰, while δ18O increases from −5.6‰ to −5.0‰.
