Llandovery epoch

In the geological timescale, the Llandovery epoch (from 443.8 ± 1.5 million years ago to 433.4 ± 0.8 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.

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.^[7]

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.^[7]

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,^[7] eliminating and changing diversity.

GSSP

The epoch was named after Llandovery in Wales.^[8] 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 (formations) occur near the boundary.^[8] The lower is the Hartfell Shale (48m thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites.^[8] Above this is the 43m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites.^[6]

The base was originally defined as the first appearance of the graptolite Akidograptus ascensus^[9] at Dob's Linn, but was later discovered to be imprecise.^{[6] [10]} It is currently placed between acritarch biozone 5 and last appearance of Pterospathodus amorphognathoides.^[6]

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.^[6]

Subdivisions

The Llandovery epoch is subdivided into three stages: Rhuddanian, Aeronian and Telychian.

Regional stages

In North America a different suite of regional stages is sometimes used:

• Ontarian (Early Silurian: late Llandovery)
• Alexandrian (Earliest Silurian: early Llandovery)

In Estonia the following suite of regional stages is used:^[11]

• Adavere stage (Early Silurian: late Llandovery)
• Raikküla stage (Early Silurian: middle Llandovery)
• Juuru stage (Earliest Silurian: early Llandovery)

Palaeontology

Agnathans of the Llandovery
Taxa	Presence	Location	Description	Images
Jamoytius	Rhuddanian-Telychian		It had an elongated body and a dorsal fin and an anal fin near the back third of its body.	Jamoytius kerwoodi

Cephalopods of the Llandovery
Taxa	Presence	Location	Description	Images
Cameroceras	Dapingian-Homerian	The shallow seas of Laurentia, Baltica and Siberia.[12]	Head was soft muscular tissue at the opening of hard cone-like shell.	Cameroceras, shown feeding on an Aphetoceras, while a quartet of Cyclostomiceras swim by.

Plants

Spores and plant microfossils have been found in China and Pennsylvania.^[13][14] 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.

Land animals

Parioscorpio venator was described as the earliest fossil land animal in 2020. It is an early scorpion (437 million years old) with respiratory structures that indicate that it was able to live on land.^[15]

Reef expansion

Barrier reef 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.^[16]

Ireviken event

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

The event lasted around 200,000 years, spanning the base of the Wenlock epoch.^[2][17]

It comprises eight extinction "datum points"—the first four being regularly spaced, every 31,000 years, and linked to the Milankovic obliquity cycle.^[17] The fifth and sixth probably reflect maxima in the precessional cycles, with periods of around 16.5 and 19 ka.^[17] The final two data are much further spaced, so harder to link with Milankovic changes.^[17]

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.^[2]

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‰.^[2]

References

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