Llandovery epoch

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.

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.[3]

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.[3]

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,[3] eliminating and changing diversity.

GSSP

The epoch was named after Llandovery in Wales.[4] 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.[4] The lower is the Hartfell Shale (48m thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites.[4] Above this is the 43m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites.[5]

The base is identified by the appearance of the graptolites Parakidograptus acuminatus and Akidograptus ascensus[6] at Dob's Linn.

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:[7]

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

Palaeontology

Jamoytius kerwoodi
Jamoytius kerwoodi
Neotype fossil (BU55) of the encrinurine trilobite Balizoma variolaris
Balizoma variolaris
Cameroceras trentonese
Cameroceras, shown feeding on a Aphetoceras, while a quartet of Cyclostomiceras swim by.

Plants

Spores and plant microfossils have been found in China and Pennsylvania.[9][10] 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.

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.[11]

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][12]

It comprises eight extinction "datum points"—the first four being regularly spaced, every 30,797 years, and linked to the Milankovic obliquity cycle.[12] The fifth and sixth probably reflect maxima in the precessional cycles, with periods of around 16.5 and 19 ka.[12] The final two data are much further spaced, so harder to link with Milankovic changes.[12]

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. ^ a b c d 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. ^ a b c 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.CS1 maint: multiple names: authors list (link)
  4. ^ a b c Gradstein, Felix M.; Ogg, James G.; Smith, Alan G. (2004). A Geologic Time Scale 2004. ISBN 9780521786737.
  5. ^ "GSSP for the Rhuddanian Stage". International Commission on Stratigraphy.
  6. ^ "Silurian: Stratigraphy". UCMP Berkeley. Retrieved 9 June 2019.
  7. ^ "Silurian Stratigraphy Of Estonia 2015" (PDF). Stratigraafia.info. Retrieved 3 February 2019.
  8. ^ 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
  9. ^ 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.
  10. ^ 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.
  11. ^ 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.
  12. ^ 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.
Aberystwyth Grits Group

The Aberystwyth Grits Group is a Silurian lithostratigraphic group (a sequence of rock strata) in mid Wales. The name is derived from Aberystwyth in northern Ceredigion where the strata are well exposed in coastal cliffs. The Group comprises the Trefechan Formation and the underlying Mynydd Bach Formation. The rocks of the Aberystwyth Grits Group have also previously been known as the Aberystwyth Grits Formation

Adelophthalmidae

Adelophthalmidae (the name deriving from the type genus Adelophthalmus, meaning "no obvious eyes") is a family of eurypterids, an extinct group of aquatic arthropods. Adelophthalmidae is the only family classified as part of the superfamily Adelophthalmoidea, which in turn is classified within the infraorder Diploperculata in the suborder Eurypterina.

Adelophthalmid eurypterids were small and swimming eurypterids that appeared in the Silurian period. With the earliest known members of the group, Nanahughmilleria prominens and Parahughmilleria maria, being known from deposits of Early Silurian (possibly the Llandovery epoch) age and the very last members, belonging to the long-lasting and widespread genus Adelophthalmus, going extinct in the Early Permian, the Adelophthalmidae is the longest lasting single family of eurypterids. The survival of the group, and of swimming eurypterids (the suborder Eurypterina) beyond the Late Devonian is entirely due to the survival, and subsequent success, of Adelophthalmus throughout the Devonian and Carboniferous. Adelophthalmus (and possibly Unionopterus) represents the only known genus of swimming eurypterids beyond the extinction of the rest of the group in the Late Devonian, extending the temporal range of the group by over a hundred million years.

Though the last swimming eurypterids and the final members of the traditionally more successful and numerous suborder Eurypterina, the adelophthalmids were not the last eurypterids. The stylonurines or the "walking eurypterids" were the last ones, surviving in the family Hibbertopteridae until the Permian-Triassic extinction event or shortly before a few million years after the extinction of the adelophthalmids in the Early Permian.

Aeronian

In the geologic timescale, the Aeronian is the age of the Llandovery epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon that began 440.8 ± 1.2 Ma and ended 438.5 ± 1.1 Ma (million years ago). The Aeronian age succeeds the Rhuddanian age and precedes the Telychian age, all in the same epoch.

Bronnant Fault

The Bronnant Fault is a geological fault affecting the lower Palaeozoic rocks of the counties of Ceredigion and Pembrokeshire in West Wales. The feature is mapped over part of its length as the Bronnant Fault Zone and is closely associated with the Glandyfi Lineament, a similarly aligned zone of faulting and folding. The rocks of the Aberystwyth Grits Group dating from the Llandovery epoch of the Silurian period thicken markedly to the west of the fault suggesting it was active at the time of their deposition. North of Lledrod its alignment is affected by cross-cutting east-west faults, including the Ystwyth Fault Zone. Its alignment is sub-parallel to the coast of Cardigan Bay thus curving from a north-south alignment in the north through a northeast-southwest alignment until it merges with broadly east-west aligned faults in northeast Pembrokeshire.

Carcinosoma

Carcinosoma (meaning "crab body") is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Carcinosoma are restricted to deposits of late Silurian (Late Llandovery to Early Pridoli) age. Classified as part of the family Carcinosomatidae, which the genus lends its name to, Carcinosoma contains seven species from North America and Great Britain.

Carcinosomatid eurypterids had unusual proportions and features compared to other eurypterids, with a broad abdomen, thin and long tail and spined and forward-facing walking appendages. They were not as streamlined as other groups but had considerably more robust and well developed walking appendages. In Carcinosoma, these spined walking appendages are thought to have been used to create a trap to capture prey in. The telson (final segment of the body) of Carcinosoma appears to have possessed distinct segmentation, Carcinosoma is the only known eurypterid to possess this feature.

At 2.2 meters (7.2 ft) in length, the species C. punctatum is the largest carcinosomatoid eurypterid by far and is among the largest eurypterids overall, rivalling the large pterygotid eurypterids (such as Jaekelopterus) in size. Other species of the genus were considerably smaller, with most ranging from 70 centimeters (2.3 ft) to 100 centimeters (3.3 ft) in length.

Claerwen Group

The Claerwen Group is a Silurian lithostratigraphic group (a sequence of rock strata) in mid Wales. The name is derived from Claerwen in Powys where the strata are exposed. The Group comprises the Rhayader Mudstones Formation and the underlying Derwenlas Formation which outcrop across the region. The rocks of the Rhayader Mudstone Formation have variously been known as the Rhayader Pale Shales, Rhayader Pale Shales Formation and Cwmsymlog Formation.

Cwmystwyth Grits Group

The Cwmystwyth Grits Group is a Silurian lithostratigraphic group (a sequence of rock strata) in mid Wales. The name is derived from the village of Cwmystwyth near Devil's Bridge in Ceredigion. The Group comprises the Blaen Myherin Mudstones Formation, the Glanyrafon Formation (upper and lower tongues), the Caerau Mudstones Formation, the Rhuddnant Grits Formation (including the Llyn Teifi Member) and the Pysgotwr Grits Formation.

Dob's Linn

Dob's Linn is a small steep valley in Dumfries and Galloway, just north of the A708 road between Moffat and Selkirk, in Scotland. It is part of the Grey Mare's Tail Nature Reserve which is owned by the National Trust for Scotland. According to tradition, Dob's Linn is named for a covenanter, Halbert Dobson, who took refuge there from Government troops during The Killing Time in the late 17th Century.

Dob's Linn is important in geology as the location of the Global Boundary Stratotype Section and Point (GSSP) which marks the boundary between the Ordovician and Silurian periods, and marks the base of the Llandovery epoch, on the geologic time scale. Dob's Linn was ratified as the GSSP by the International Union of Geological Sciences in 1984.The boundary is defined as the first appearance of graptolites Parakidograptus acuminatus and Akidograptus ascensu 1.6 m above the base of the Birkhill Shale Formation. The shale section also contains chitinozoa and conodonts, but neither are well preserved. Dob's Linn has been criticized for the difficulty in relating its graptolite biostratigraphic sequence with shallow water sequences elsewhere, although the stratotype also appears to correspond with a carbon-13 isotope excursion in the latest Ordovician which can be identified worldwide.The area was first studied by Charles Lapworth in the late 19th century. His work established fossil graptolites as a method of understanding stratigraphic sequences. A more recent description of the area, enumerating the zones established by Lapworth, is given in the British Regional Geology Monograph. Before Lapworth's work, it was thought that the Silurian rocks of the Southern Uplands formed a single sequence, that would have to be about 6000m in thickness. By his identification of particular graptolite species in different zones of the Dob's Linn exposure, Lapworth was able to demonstrate that the Uplands consist of a much thinner layer, consistent with Silurian deposits elsewhere, that had been repeatedly folded and faulted, with multiple repetitions of the same strata, often upside down. The understanding that sequences of sedimentary rocks could be inverted played an important part in the later resolution of the Highlands Controversy in which Lapworth was also involved.

Hutton's Unconformity

Hutton's Unconformity is a name given to various notable geological sites in Scotland identified by the 18th-century Scottish geologist James Hutton as places where the junction between two types of rock formations can be seen. This geological phenomenon marks the location where rock formations created at different times and by different forces adjoin. For Hutton, such an unconformity provided evidence for his Plutonist theories of uniformitarianism and the age of the Earth.

An unconformity is any break in the normal progression of sedimentary deposits, which are laid the newer on top of the older.

In his search, Hutton and colleagues examined rock outcrops and cliffs, both riverside and sea, and found several locations where two adjoining rock types had been laid bare, the most noted being at Siccar Point on the coast of Berwickshire.

Jamoytius

Jamoytius kerwoodi was a species of primitive, eel-like jawless fish that lived in the Llandovery epoch of the Early Silurian period.

Long thought of as a "basal anaspid," J. kerwoodi is now recognized as the best-known member of the Hyperoartian order Jamoytiiformes. It had an elongated body, and is thought to have had, in comparison with relatives known from intact bodies like Euphanerops, a dorsal fin and an anal fin near the rearmost third of its body. Earlier reconstructions depict the creature as having side-fins running the length of its body, starting from behind the branchial openings to the tip of its tail: new research demonstrates that such "fins" are actually deformations of the bodywall as the corpse was being squished post-burial. In life, J. kerwoodi resembled a lamprey with a very small mouth. Because the fossil had no teeth, teeth-like structures, nor suggestions of either in its mouth, it was not carnivorous like many modern lampreys. It was more likely to have been a filter-feeder or a detritus-feeder, possibly in the manner of larval lampreys.

The fish had a cartilaginous skeleton, and a branchial basket resembling the cyclostomes - features that suggest that it was a basal member of that clade. It is also the earliest known vertebrate with camera-type eyes. It also possessed weakly mineralised scales.

Llandovery

Llandovery (, [ɬan-]; Welsh: Llanymddyfri [ɬanəmˈðəvrɪ]) is a market town and community in Carmarthenshire, Wales. It lies on the River Tywi and the junction of the A40 and A483 roads, around 25 miles (40 km) northeast of Carmarthen and 27 miles (43 km) north of Swansea.

Llandovery's Welsh name is derived from Llan ymlith y dyfroedd, meaning "church enclosure amidst the waters" and referring to the town's position between the River Tywi and the Afon Brân just upstream of their confluence. A smaller watercourse, the Bawddwr, runs through and under the town.

The town is served by Llandovery railway station, on the Heart of Wales line, with services to Swansea and Shrewsbury. Llandovery is twinned with Pluguffan in Brittany, France.

Onychopterella

Onychopterella ( ON-ə-kop-tə-REL-ə, from Ancient Greek: ὄνῠξ (ónux), "claw", and πτερόν (pteron), "wing") is a genus of predatory eurypterid ("sea scorpion"), an extinct group of aquatic arthropods. Fossils of Onychopterella have been discovered in deposits from the Late Ordovician to the Late Silurian. The genus contains three species: O. kokomoensis, the type species, from the Early Pridoli epoch of Indiana; O. pumilus, from the Early Llandovery epoch of Illinois, both from the United States; and O. augusti, from the Late Hirnantian to Early Rhuddanian stages of South Africa.

Its prosoma (head) could be subquadrate (almost square) or subrectangular (almost rectangular), with reniform (bean-shaped) eyes. The appendages (limbs) were generally long and narrow with a spine on their tip. The abdomen and telson ("tail") had different shapes and sizes depending on the species. The ornamentation of the body consisted of small, pointed scales. The largest species of the genus was O. kokomoensis with a total length of 16 centimetres (6.3 inches) long, followed by O. augusti (14.3 cm, 5.6 in) and O. pumilus (4 cm, 1.6 in).

The first Onychopterella fossils, belonging to O. kokomoensis, were discovered in 1896 at the Waterlime Group of Kokomo, Indiana. It has received attention from eurypterid researchers for its terminal claw in the sixth pair of appendages or swimming legs. Onychopterella is also the type genus of the basal ("primitive") family of eurypterines Onychopterellidae together with Alkenopterus and Tylopterella, characterized by the presence of spines on the second to fourth pair of appendages and a lack of them on the fifth and sixth pair of appendages (except occasionally one on the distal end of the swimming leg), as well as the lanceolate (lance-shaped) or styliform (pen-shaped) form of the telson and other characteristics.

The exceptional preservation of the fossils of O. augusti has permitted scientists to describe part of the alimentary canal that has only been observed in a few species of eurypterids, as well as the internal muscular structure of its limbs and even part of the external branchial respiratory system. This turned out to be highly similar to that of the scorpions of today, supporting a eurypterid-scorpion relationship. Onychopterella was a genus that was able to swim. Most of the time it was likely in the stratum, probably using its spines to walk and its head to dig in the ground.

Parys Mountain

Parys Mountain (Welsh: Mynydd Parys) is located south of the town of Amlwch in north east Anglesey, Wales. It is the site of a large copper mine that was extensively exploited in the late 18th century.

Placodermi

Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. Placoderms are paraphyletic, and consist of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil, Entelognathus, from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods. The jaws in other placoderms were simplified and consisted of a single bone. Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth. Paraphyletic groupings are problematic, as one can not talk precisely about their phylogenic relationships, their characteristic traits and literal extinction. 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.In contrast, one 2016 analysis concluded that placodermi are likely monophyletic.The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian. The various groups of placoderms were diverse and abundant during the Devonian, but became extinct at the end-Devonian Hangenberg event 358.9 million years ago

Red Mountain Expressway Cut

The Red Mountain Expressway Cut, also known as the Red Mountain Geological Cut, is a section of Red Mountain that was blasted and removed in the 1960s to allow the Red Mountain Expressway to enter downtown Birmingham, Alabama. This highway links Birmingham with its southern suburbs of Homewood, Mountain Brook, and Vestavia Hills. It has spurred suburban growth towards the south of Birmingham. This section also provides the route for U.S. Route 31 (US 31) to the south (the Montgomery Highway) and US 280 to the southeast.

The resultant cut exposes geological strata spanning millions of years (150 million years of geological time within 650 feet or 200 metres of exposure), including the red ore seam that spurred Birmingham's development. A new species of Lower Silurian (middle Llandovery epoch) phacopsid trilobite, Acaste birminghamensis, was first collected from exposures on Red Mountain. Named for the city, the new species was published in May 1972.The cut was designated a National Natural Landmark in 1987.

Rhuddanian

In the geologic timescale, the Rhuddanian is the first age of the Silurian period and of the Llandovery epoch. The Silurian is in the Paleozoic era of the Phanerozoic eon. The Rhuddanian age began 443.7 ± 1.5 Ma and ended 440.8 ± 1.2 Ma (million years ago). It succeeds the Himantian Age (the last age of the Ordovician Period) and precedes the Aeronian age.

Stockdale Group

The Stockdale Group is a Silurian lithostratigraphic group (a sequence of rock strata) in the southern Lake District and Howgill Fells of the Pennines of northern England. The name is derived from the locality of Stockdale near the top of Longsleddale in Cumbria. It is included within the Windermere Supergroup. The rocks of the Group have also previously been referred to as the Stockdale Shales or Stockdale Subgroup. The group comprises limestones and oolites and some sandstones and shales which reach a maximum thickness of 120m in the area. It is divided into a lower Skelgill Formation which is overlain by an upper Browgill Formation.

Telychian

In the geologic timescale, the Telychian is the age of the Llandovery epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon that is comprehended between 436.0 ± 1.9 Ma and 428.2 ± 2.3 Ma (million years ago), approximately. The Telychian age succeeds the Aeronian age and precedes the Sheinwoodian age.It ended with the Ireviken event.

Wenlock epoch

The Wenlock (sometimes referred to as the Wenlockian) is the second series of the Silurian. It is preceded by the Llandovery epoch and followed by the Ludlow Group. Radiometric dates constrain the Wenlockian between 433.4 and 427.4 million years ago.

Cenozoic era
(present–66.0 Mya)
Mesozoic era
(66.0–251.902 Mya)
Paleozoic era
(251.902–541.0 Mya)
Proterozoic eon
(541.0 Mya–2.5 Gya)
Archean eon (2.5–4 Gya)
Hadean eon (4–4.6 Gya)

Languages

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