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.[3]

Naming and history

The Wenlock is named after Wenlock Edge, an outcrop of rocks near the town of Much Wenlock in Shropshire (West Midlands, United Kingdom).[4] The name was first used in the term "Wenlock and Dudley rocks" by Roderick Murchison in 1834 to refer to the limestones and underlying shales that underlay what he termed the "Ludlow rocks".[5] He later modified this term to simply the "Wenlock rocks" in his book, The Silurian System in 1839.[6]

Definition and subdivision

The Wenlock's beginning is defined by the lower boundary (or GSSP) of the Sheinwoodian. The end is defined as the base (or GSSP) of the Gorstian.[7]

The Wenlock is divided into the older Sheinwoodian and the younger Homerian stage. The Sheinwoodian lasted from 433.4 to 430.5 million years ago. The Homerian lasted from 430.5 to 427.4 million years ago.[3]

Paleontology

Arthropods

Arctinurus boltoni
Arctinurus boltoni

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. ^ a b "International Chronostratigraphic Chart 2013/01" (PDF). International Commission on Stratigraphy. Retrieved 10 July 2013.
  4. ^ Gradstein, Felix M.; Ogg, James G.; Smith, Alan G. (2004). A Geologic Time Scale 2004. ISBN 9780521786737.
  5. ^ Murchison R.I. (1833–1834). "On the Structure and Classification of the Transition Rocks of Shropshire, Herefordshire and part of Wales and on the Lines of Disturbance which have affected that Series of Deposits, including the Valley of Elevation of Woolhope". Proceedings of the Geological Society of London. 2 (33): 14.
  6. ^ Murchison R.I. (1839). The Silurian System. Murray. p. 208.
  7. ^ The Geologic Time Scale 2012. Elsevier Science Ltd. ISBN 978-0-444-59425-9.
Arctinurus

Arctinurus boltoni is a large (up to 30 cm) lichid trilobite of the mid-Silurian. This trilobite reached about eight inches in length, though the normal adult carapace was about four inches. It lived in moderately deep-water in semi-tropical regions. Arctinurus fossils have been found in Europe and North America. Arctinurus was first reported during the construction of the Erie Canal through soft Silurian shales and mudstones in upstate New York. Before the late 1990s, complete Arctinurus fossils were very rare. The vast majority of complete specimens were commercially mined near Middleport New York, USA, in a shallow quarry in the Rochester Formation, and the trilobite is now relatively common in museum, university and private collections. Arctinurus tended to have epibionts attached to the carapace.

Chondrichthyes

Chondrichthyes (; from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') is a class that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).

Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

Cooksonia

Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian (the Wenlock epoch); the group continued to be an important component of the flora until the end of the Early Devonian, a total time span of 433 to 393 million years ago. While Cooksonia fossils are distributed globally, most type specimens come from Britain, where they were first discovered in 1937. Cooksonia includes the oldest known plant to have a stem with vascular tissue and is thus a transitional form between the primitive non-vascular bryophytes and the vascular plants.

Dalby Group

The Dalby Group is a Silurian lithostratigraphic group (a sequence of rock strata) on the west coast of the Isle of Man in the Irish Sea. The name is derived from the village of Dalby near the west coast of the island. Together with those of the adjoining Manx Group, the rocks of the Group have also previously been referred to as the Manx Slates or Manx Slate Series. The group comprises wacke sandstones with siltstones and mudstones which reach a thickness of about 1200m in the west of the island. It contains only the single Niarbyl Formation which is exposed along the coast between Niarbyl Point and the town of Peel to the north.

Galeaspida

Galeaspida (from Latin, "Helmet shields") is an extinct taxon of jawless marine and freshwater fish. The name is derived from galea, the Latin word for helmet, and refers to their massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. However, Galeaspida are in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.

Geology of Northumberland

This article describes the geology of the historic county of Northumberland. It does not include that southeastern part of the historic county which has since 1974 formed a part of the metropolitan county of Tyne and WearThe geology of Northumberland in northeast England includes a mix of sedimentary, intrusive and extrusive igneous rocks from the Palaeozoic and Cenozoic eras. Devonian age volcanic rocks and a granite pluton form the Cheviot massif. The geology of the rest of the county is characterised largely by a thick sequence of sedimentary rocks of Carboniferous age. These are intruded by both Permian and Palaeogene dykes and sills and the whole is overlain by unconsolidated sediments from the last ice age and the post-glacial period. The Whin Sill makes a significant impact on Northumberland's character and the former working of the Northumberland Coalfield significantly influenced the development of the county's economy. The county's geology contributes to a series of significant landscape features around which the Northumberland National Park was designated.

Homerian

In the geologic timescale, the Homerian is the age of the Wenlock epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon that is comprehended between 426.2 ± 2.4 Ma and 422.9 ± 2.5 Ma (million years ago), approximately. The Homerian age succeeds the Sheinwoodian age and precedes the Gorstian age.

The name comes from the small village of Homer, Shropshire near Much Wenlock.

Ireviken event

The Ireviken event was the first of three relatively minor extinction events (the Ireviken, Mulde, and Lau events) during the Silurian period. It occurred at the Llandovery/Wenlock boundary (mid Silurian, 433.4 ± 2.3 million years ago). The event is best recorded at Ireviken, Gotland, where over 50% of trilobite species became extinct; 80% of the global conodont species also become extinct in this interval.

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.

Ludlow epoch

In the geological timescale, the Ludlow epoch (from 422.9 ± 2.5 million years ago to 418.7 ± 2.7 million years ago) occurred during the Silurian period, after the end of the Homerian age. It is named for the town of Ludlow in Shropshire, England.

The Ludlow epoch is subdivided into two stages: Gorstian and Ludfordian.

Much Wenlock Limestone Formation

The Much Wenlock Limestone Formation is a series of Silurian limestone beds that date back to the Homerian age, the early part of the Wenlock epoch. The formation comprises nodular to thinly bedded limestones, with variable development of reef bodies that can be found exposed at Wenlock Edge and in a small area near Dudley, England. The formation is made up of three different members which have been classified as the Lower Quarried Limestone Member, the Nodular Beds Member and the Upper Quarried Limestone Member. Many of the members can be subdivided into separate lithofacies that would have been deposited under different environmental conditions. However the overall environment of deposition for the formation is in a shallow, tropical marine setting that is located between the storm wave base and the fair weather wave base.

New Zealand geologic time scale

While also using the international geologic time scale, many nations - especially those with isolated and therefore non-standard prehistories - use their own system of dividing geologic time into epochs and faunal stages.

In New Zealand, these epochs and stages use local place names (mainly Maori in origin) back to the Permian. Prior to this time, they largely use the same terms as used in the Australian geologic time scale, and are not divided into epochs. In practice, these early terms are rarely used, as most New Zealand geology is of more recent origin. In all cases, New Zealand uses the same periods as used internationally; it is only the subdivisions of these periods that have been renamed. Very few epochs and stages cross international period boundaries. Of those that do, almost all are within the Cenozoic Era.

Though the New Zealand geologic time scale has not been formally adopted, it has become widely used by New Zealand earth scientists, geologists and palaeontologists since its proposal by J. S. Crampton in 1995.

A standard abbreviation is also used for these epochs and stages, mostly in the form Xx where the first letter is the initial letter of the epoch and the second (lower-case) letter is the initial letter of the stage. These are listed alongside the stage names in the list below.

Currently, we are in the Haweran stage of the Wanganui epoch. The Haweran, which started some 340,000 years ago, is named after the North Island town of Hawera.

Pneumodesmus

Pneumodesmus newmani is a species of millipede that lived in the Paleozoic. Its exact age is uncertain. It was originally interpreted as living 428 million years ago, in the Late Silurian; however, the study conducted by Suarez et al. (2017) indicates that it actually lived in the Early Devonian (Lochkovian). It is the first myriapod, and the oldest known creature to have lived on land. It was discovered in 2004, and is known from a single specimen from Stonehaven, Aberdeenshire, Scotland.

Sheinwoodian

In the geologic timescale, the Sheinwoodian is the age of the Wenlock epoch of the Silurian period of the Paleozoic era of the Phanerozoic eon that is comprehended between 428.2 ± 2.3 Ma and 426.2 ± 2.4 Ma (million years ago), approximately. The Sheinwoodian age succeeds the Telychian age and precedes the Homerian age.

Similodonta

Similodonta is an extinct genus of early bivalve in the extinct family Praenuculidae. The genus is one of eleven genera in the subfamily Praenuculinae. Similodonta is known from Middle Ordovician through Middle Silurian fossils found in Europe and North America. The genus currently contains eight accepted species, Similodonta ceryx, Similodonta collina, Similodonta djupvikensis, Similodonta magna, Similodonta recurva, Similodonta spjeldnaesi, Similodonta wahli and the type species Similodonta similis.

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.

Tranearth Group

The Tranearth 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 Tranearth near Torver in Cumbria. The Group is included within the Windermere Supergroup. It comprises laminated hemipelagites and siltstones and some sandstones and limestone which achieve a thickness of between 500 and 1000m. It is divided into a lower Brathay Formation which is overlain by the Birk Riggs Formation (not present in the Howgills) and then by an upper Coldwell Formation.

Wenlock Edge

Wenlock Edge is a limestone escarpment near Much Wenlock, Shropshire, England and a site of special scientific interest because of its geology. It is over 19 mi (31 km) long, running southwest to northeast between Craven Arms and Much Wenlock, and is roughly 1,083 feet above sea level. The deciduous woodland which runs along it covers much of the steep slopes of the escarpment and in parts it is very well preserved.

It was featured on the 2005 TV programme Seven Natural Wonders as one of the wonders of the Midlands.

Wenlock Edge contains many interesting features such as Flounder's Folly, Wilderhope Manor and Shipton Hall and waymarked walks such as the Shropshire Way and bridleways such as the Jack Mytton Way. It is a popular area for hillwalking, cycling, mountain biking and horseriding and is also frequented by tourists and sightseers.

Robert Hart created a model forest garden from a small orchard on his farm called Highwood Hill in Wenlock Edge.

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)

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