Llandovery Epoch

In the geological timescale, the Llandovery Epoch (from 443.1 ± 0.9 million years ago to 432.9 ± 1.2 million years ago) occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.

Widespread reef building started in this period and continued into the Devonian Period when rising water temperatures are thought to have bleached out the coral by killing their photo symbionts.

The Llandoverian Epoch ended with the Ireviken event which killed off 50% of trilobite species, and 80% of the global conodont species.

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 (southern Scotland) in an artificial excavation created just north of the Linn Branch Stream. Two lithological units (formations) occur near the boundary. The lower is the 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.