In the geological timescale, the Llandovery epoch (from 443.7 ± 1.5 million years ago to 428.2 ± 2.3 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.
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.
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 (formations) occur near the boundary. The lower is the Hartfell Shale (48m thick), 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.
In North America a different suite of regional stages is sometimes used:
- Ontarian (Early Silurian: late Llandovery)
- Alexandrian (Earliest Silurian: early Llandovery)
- Adavere stage (Early Silurian: late Llandovery)
- Raikküla stage (Early Silurian: middle Llandovery)
- Juuru stage (Earliest Silurian: early Llandovery)
|Agnathans of the Llandovery|
|Jamoytius||Rhuddanian-Telychian||It had an elongated body and a dorsal fin and an anal fin near the back third of its body.|
|Arthropods of the Llandovery|
|Balizoma||440.8 million years ago to Pridoli epoch||Dudley, England||only been found in rocks of Silurian age.|
|Cephalopods of the Llandovery|
|Cameroceras||Dapingian-Homerian||The shallow seas of Laurentia, Baltica and Siberia.||Head was soft muscular tissue at the opening of hard cone-like shell.|
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 (Cooksonia) have only been found in rocks of the middle Silurian.
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.
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
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.
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.
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- 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.
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- 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
- 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.
- 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.
- 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.
- 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.