| Llandovery | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 443.1 ± 0.9 – 432.9 ± 1.2Ma | |||||||||||||
.png) A map of Earth as it appeared 440 million years ago during the Llandovery Epoch, Aeronian Age | |||||||||||||
| Chronology | |||||||||||||
| |||||||||||||
| Etymology | |||||||||||||
| Name formality | Formal | ||||||||||||
| Name ratified | 1984 | ||||||||||||
| Usage information | |||||||||||||
| Celestial body | Earth | ||||||||||||
| Regional usage | Global (ICS) | ||||||||||||
| Time scale(s) used | ICS Time Scale | ||||||||||||
| Definition | |||||||||||||
| Chronological unit | Epoch | ||||||||||||
| Stratigraphic unit | Series | ||||||||||||
| Time span formality | Formal | ||||||||||||
| Lower boundary definition | FAD of theGraptoliteAkidograptus ascensus | ||||||||||||
| Lower boundary GSSP | Dob's Linn,Moffat, UK 55°26′24″N3°16′12″W / 55.4400°N 3.2700°W /55.4400; -3.2700 | ||||||||||||
| Lower GSSP ratified | 1984[4][5] | ||||||||||||
| Upper boundary definition | Imprecise. Currently placed between acritarch biozone 5 and last appearance ofPterospathodus amorphognathoides.See text for more info. | ||||||||||||
| Upper boundary definition candidates | A conodont boundary (Ireviken datum 2) which is close to themurchisoni graptolite biozone. | ||||||||||||
| Upper boundary GSSP candidate section(s) | None | ||||||||||||
| Upper boundary GSSP | Hughley Brook,Apedale, UK 52°34′52″N2°38′20″W / 52.5811°N 2.6389°W /52.5811; -2.6389 | ||||||||||||
| Upper GSSP ratified | 1980[6] | ||||||||||||
In thegeological timescale, theLlandovery Epoch (from 443.1 ± 0.9 million years ago to 432.9 ± 1.2 million years ago) occurred at the beginning of theSilurian Period. The Llandoverian Epoch follows the massiveOrdovician-Silurian extinction events, which led to a large decrease inbiodiversity and an opening up ofecosystems.
Widespreadreef building started in this period and continued into theDevonian Period when rising water temperatures are thought to have bleached out thecoral by killing their photosymbionts.
The Llandoverian Epoch ended with theIreviken event which killed off 50% oftrilobite species, and 80% of the globalconodont 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 theSilurian, rebounded within the survivingorders.[7]
Following the major loss of diversity as the end-Ordovician, Silurian communities were initially less complex and broader niched. Highlyendemic faunas, which characterized the Late Ordovician, were replaced by faunas that were amongst the most cosmopolitan in thePhanerozoic, biogeographic patterns that persisted throughout most of the Silurian.[7]
These end Ordovician–Silurian events had nothing like the long-term impact of thePermian–Triassic andCretaceous-Paleogene extinction events. Nevertheless, a large number of taxa disappeared from the Earth over a short time interval,[7] eliminating and changing diversity.
The epoch was named afterLlandovery in Wales.[8] TheGSSP for the Silurian is located in a section atDob's Linn (southern Scotland) in an artificial excavation created just north of the Linn Branch Stream. Twolithological units (formations) occur near the boundary.[8] The lower is theHartfell Shale (48 metres (157 ft) thick), consisting chiefly of pale gray mudstone with subordinate blackshales and severalinterbedded meta-bentonites.[8] Above this is the 43 metres (141 ft) thickBirkhill 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 thegraptoliteAkidograptus 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 ofPterospathodus 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 themurchisoni Graptolite Biozone.[6]
The Llandovery Epoch is subdivided into three stages:Rhuddanian,Aeronian andTelychian.
In North America a different suite of regional stages is sometimes used:
InEstonia the following suite of regional stages is used:[11]
Spores and plant microfossils have been found in China and Pennsylvania.[12][13] 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.
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.[14]
Barrier reef systems covered a substantially greater percentage of seafloor than reefs today and they also grew at high latitudes. Possibly the evolution of photosymbionts started in the Llandovery Epoch.Tabulate corals mostly developed as prominentbioherms. Rising water temperatures in the Devonian might have led to bleaching of these corals.[15]
The Ireviken event was the first of three relatively minorextinction events (the Ireviken,Mulde, andLau events) during the Silurian Period. The Ireviken overlapped the Llandovery/Wenlock boundary. The event is best recorded atIreviken,Gotland.
The event lasted around 200,000 years, spanning the base of the Wenlock Epoch.[2][16]
It comprises eight extinction "datum points"—the first four being regularly spaced, every 31,000 years, and linked to theMilankovic obliquity cycle.[16] The fifth and sixth probably reflect maxima in the precessional cycles, with periods of around 16.5 and 19 ka.[16] The final two data are much further spaced, so harder to link withMilankovic changes.[16]
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 andtrilobites were hit hardest. 50% of trilobite species and 80% of the global conodont species become extinct in this interval.[2]
Subsequent to the first extinctions, excursions in theδ13C andδ18O records are observed; δ13C rises from +1.4‰ to +4.5‰, while δ18O increases from −5.6‰ to −5.0‰.[2]
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