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.

Llandovery
Chronology
Etymology
Name formalityFormal
Name ratified1984
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEpoch
Stratigraphic unitSeries
Time span formalityFormal
Lower boundary definitionFAD of the Graptolite Akidograptus ascensus
Lower boundary GSSPDob's Linn, Moffat, UK
55.4400°N 3.2700°W / 55.4400; -3.2700
Lower GSSP ratified1984[4][5]
Upper boundary definitionImprecise. Currently placed between acritarch biozone 5 and last appearance of Pterospathodus amorphognathoides. See text for more info.
Upper boundary definition candidatesA conodont boundary (Ireviken datum 2) which is close to the murchisoni graptolite biozone.
Upper boundary GSSP candidate section(s)None
Upper boundary GSSPHughley Brook, Apedale, UK
52.5811°N 2.6389°W / 52.5811; -2.6389
Upper GSSP ratified1980[6]

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 (southern Scotland) 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 (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

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
  1. Jeppsson, L.; Calner, M. (2007). "The Silurian Mulde Event and a scenario for secundo—secundo events". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 93 (02): 135–154. doi:10.1017/S0263593300000377.
  2. Munnecke, A.; Samtleben, C.; Bickert, T. (2003). "The Ireviken Event in the lower Silurian of Gotland, Sweden-relation to similar Palaeozoic and Proterozoic events". Palaeogeography, Palaeoclimatology, Palaeoecology. 195 (1): 99–124. doi:10.1016/S0031-0182(03)00304-3.
  3. "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  4. Lucas, Sepncer (6 November 2018). "The GSSP Method of Chronostratigraphy: A Critical Review". Frontiers in Earth Science. 6: 191. Bibcode:2018FrEaS...6..191L. doi:10.3389/feart.2018.00191.
  5. Holland, C. (June 1985). "Series and Stages of the Silurian System" (PDF). Episodes. 8 (2): 101–103. doi:10.18814/epiiugs/1985/v8i2/005. Retrieved 11 December 2020.
  6. "GSSP for the Rhuddanian Stage". International Commission on Stratigraphy.
  7. Harper, D. A. T., Hammarlund, E. U., & Rasmussen, C. M. Ø. (May 2014). "End Ordovician extinctions: A coincidence of causes". Gondwana Research. 25 (4): 1294–1307. Bibcode:2014GondR..25.1294H. doi:10.1016/j.gr.2012.12.021.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. Gradstein, Felix M.; Ogg, James G.; Smith, Alan G. (2004). A Geologic Time Scale 2004. ISBN 9780521786737.
  9. "Silurian: Stratigraphy". UCMP Berkeley. Retrieved 9 June 2019.
  10. Ogg, James; Ogg, Gabi; Gradstein, Felix (2016). A Concise Geologic Time Scale. 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. Wang, Yi; Zhang, Yuandong (2010). "Llandovery sporomorphs and graptolites from the Manbo Formation, the Mojiang County, Yunnan, China". Proceedings of the Royal Society B: Biological Sciences. 277 (1679): 267–275. doi:10.1098/rspb.2009.0214. PMC 2842664. PMID 19439443.
  14. Strother, Paul K.; Traverse, Alfred (1979). "Plant microfossils from Llandoverian and Wenlockian rocks of Pennsylvania". Palynology. 3: 1–21. doi:10.1080/01916122.1979.9989181.
  15. Anderson, Evan P; Schiffbauer, James D.; Jacquet, Sarah M.; Lamsdell, James C.; Kluessendorf, Joanne; Mikulic, Donald G. (2021). "Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte". Palaeontology. 64 (3): 429–474. doi:10.1111/pala.12534. ISSN 1475-4983. S2CID 234812878.
  16. Zapalski, Mikołaj K.; Berkowski, Błażej (2019). "The Silurian mesophotic coral ecosystems: 430 million years of photosymbiosis". Coral Reefs. 38 (1): 137–147. Bibcode:2019CorRe..38..137Z. doi:10.1007/s00338-018-01761-w.
  17. 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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