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23-470: Armenoceras is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician , through the remainder of the period and on into the Upper Silurian . It is the type genus of the family Armenoceratidae . The shells of Armenoceras are straight and medium to large in size with a circular to subcircular cross section. The siphuncle

46-667: A second wutinocerid genus, Adamsoceras . Gonioceras is an offshoot of an early Armenoceras; Lambeoceras and Huronia are offshoots of a later Actinoceras . The Actinoceratidae extend into the Lower Silurian with Actinoceras ; the Armenoceratidae and Huroniidae extend into the Upper Silurian. The Ormoceratidae are possibly the most recent, extending into the Lower Devonian Gonioceras (Gonioceratidae)

69-596: A simplification of the endosiphuncular canal system. The earliest Armenoceras known comes from the lower Whiterock equivalent in northern China and Korea but is unknown in North America until the end of the Middle Ordovician when it appears in Red River faunas. Worldwide, in addition to east Asia and North America including Greenland, Armenoceras has been found in northern Europe, Russia, and Australia. Armenoceras

92-625: Is antedated by Wutinoceras , its assumed primitive nature rather a derived condition. Actinocerids first appeared early in the Middle Ordovician, with the exception of the Georginidae, which are known from the Cassinian in Northern Australia. They reached their greatest diversity in the Middle Ordovician with more than 20 genera, then declined somewhat in the Late Ordovician and more so in

115-622: Is best recorded at Ireviken , Gotland . The event lasted around 200,000 years, spanning the base of the Wenlock Epoch. It comprises eight extinction "datum points"—the first four being regularly spaced, every 31,000 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

138-419: Is large, located subcentrally to resting on the ventral margin. Segments are wider than long, broadly expanded into the camerae. Septal necks short with wide brings that may be in contact with posterior surface of septa. In most the canal system within the siphuncle in is of the double arc type. Cameral deposits are rare. Armenoceras is derived from Wutinoceras through a thinning of the connecting rings and

161-583: Is limited to the Middle Ordovician, its quasi-lookalike Lambeoceras (Lamberoceratidae) to the uppermost Middle and Upper Ordovician. The wutinocerids are known only from the early Middle Ordovician (Whiterockian) and the polydesmiadids are restricted to about that time. Originating in the Ordovician, by the Devonian period actinocerids became rare; perhaps they were unable to compete with the more compact and maneuverable coiled nautiloids and ammonoids and cope with

184-738: Is thought to have given rise to Nybyoceras in the Chazyan (Flower 1968), although the reverse was earlier suggested in Teichert (1964), and to Selkirkoceras in the Eden-Red River Stage in the Upper Ordovician. Possible Silurian derivatives include Megadiscosorus , Monocyrtoceras , and Elrodoceras although some may turn our to have Nybyoceras as the progenitor. Actinocerida The Actinocerida are an order of generally straight, medium to large cephalopods that lived during

207-534: Is unclear. The Upper Cambrian Protactinocerida have been suggested as being ancestral but none are known to have gone beyond the near end of the Cambrian extinction, which makes any connection hypothetical. Polydesmia was once thought to be the ancestral form of the actinocerids, and was derived from the ellesmeroceriid Bathmoceras . However, it turned out based on a reassessment of Lower Ordovician and Whiterockian formations in northeastern China that Polydesmia

230-605: 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. The end of the Ordovician–Silurian extinction event occurred when melting glaciers caused the sea level to rise and eventually stabilize. Biodiversity, with

253-735: The Early Silurian ; made a slight come back in the Middle Silurian but not to Late Ordovician numbers; and declined more or less steadily from the Late Silurian into the Devonian. Three major lineages began the Middle Ordovician, the Actinoceratidae, Armenoceratidae, and Ormoceratidae. The Actinoceratidae and Armenoceratidae are most likely derived from Wutinoceras and the Ormoceratidae from

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276-465: The Georginidae but don't become well established until the beginning of the early Whiterockian Stage (Dapingian) of the Middle Ordovician (Flower 1868,1976) The Georginidae, introduced and described by Mary Wade in 1977 (Wade 1988), based on the genus Georgina , are known from the upper Canadian Coolibah Formation of the Georgina Basin in Northern Australia. How the Georginidae relate to older stocks

299-641: The Huroniidae of the Silurian grew significantly larger. The Actinocerida inhabited shallow to quite deep waters, where they alternated between swimming and lying on the bottom. They were predatory, and able to control their buoyancy to a greater degree than their contemporaries. The derivation of the Actinocerida remains enigmatic. They first appear late in the Early Ordovician (Cassinian Stage, late Floian) with

322-579: The Linn Branch Stream. Two lithological units ( formations ) occur near the boundary. 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 . 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. The base

345-559: 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. Barrier reef systems covered a substantially greater percentage of seafloor than reefs today and they also grew at high latitudes. Possibly

368-637: 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 (southern Scotland) in an artificial excavation created just north of

391-540: The arrival of jawed fish. The Actinocerida contain nine families; the Georginidae, Wutinoceratidae, Polydesmiidae, Armenoceratidae, Ormoceratidae, Actinoceratidae, Gonioceratidae, Lambeoceratide, and Huroniidae. The Carbactinoceratidae, included in the taxonomy in the Treatise, (Vol K) have been removed to the Pseudorthocerida. Llandovery epoch Widespread reef building started in this period and continued into

414-454: The base of the murchisoni Graptolite Biozone. The Llandovery Epoch is subdivided into three stages: Rhuddanian , Aeronian and Telychian . In North America a different suite of regional stages is sometimes used: In Estonia the following suite of regional stages is used: Spores and plant microfossils have been found in China and Pennsylvania. There was some movement to the land during

437-479: The early and middle Paleozoic , distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial canals and a narrow space along the inner side of the connecting ring known as a paraspatium. (Teichert 1964) Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm (2.0 to 3.0 ft) but some, like

460-410: 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. Subsequent to the first extinctions, excursions in

483-662: 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

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506-566: 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

529-459: Was originally defined as the first appearance of the graptolite Akidograptus ascensus at Dob's Linn, but was later discovered to be imprecise. It is currently placed between acritarch biozone 5 and last appearance of Pterospathodus amorphognathoides . It has been recommended to place the GSSP at a slightly higher and correlatable level on the Ireviken datum 2, which coincides approximately with

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