Llandovery Epoch

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Llandovery
443.8 ± 1.5 – 433.4 ± 0.8 Ma
Chronology

In the

ecosystems
.

Widespread

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

Hartfell Shale (48 metres (157 ft) thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites.[8] 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.[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 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.[15]

Reef expansion

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 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

  1. doi:10.1017/S0263593300000377
    .
  2. ^
    doi:10.1016/S0031-0182(03)00304-3
    .
  3. ^ "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  4. doi:10.3389/feart.2018.00191
    .
  5. doi:10.18814/epiiugs/1985/v8i2/005
    . Retrieved 11 December 2020.
  6. ^ a b c d e "GSSP for the Rhuddanian Stage". International Commission on Stratigraphy.
  7. ^
    doi:10.1016/j.gr.2012.12.021
    .
  8. ^
    ISBN 9780521786737
    .
  9. ^ "Silurian: Stratigraphy". UCMP Berkeley. Retrieved 9 June 2019.
  10. ISBN 978-0-444-63771-0
    .
  11. ^ "Silurian Stratigraphy Of Estonia 2015" (PDF). Stratigraafia.info. Retrieved 3 February 2019.
  12. ^ Frey, R.C. 1995. "Middle and Upper Ordovician nautiloid cephalopods of the Cincinnati Arch region of Kentucky, Indiana, and Ohio" (PDF). U.S. Geological Survey, p.73
  13. PMID 19439443
    .
  14. doi:10.1080/01916122.1979.9989181
    .
  15. S2CID 234812878
    .
  16. doi:10.1007/s00338-018-01761-w
    .
  17. ^ a b c d Jeppsson, L (1997). "The anatomy of the Mid-Early Silurian Ireviken Event and a scenario for P-S events". In Brett, C.E.; Baird, G.C. (eds.). Paleontological Events: Stratigraphic, Ecological, and Evolutionary Implications. New York: Columbia University Press. pp. 451–492.
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