Trinodus
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''Trinodus'' is a very small to small (about ) blind
trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...
, a well known group of extinct marine
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s, which lived during the
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
(Tremadocian to early Hirnantian), in what are now the Yukon Territories, Virginia, Italy, Czech Republic, Poland, Denmark, Sweden, Svalbard, Ireland, Scotland, Wales, Iran, Kazakhstan and China. It is one of the last of the Agnostida order to survive.


Etymology

''Trinodus'' is derived from the Latin tri (three) and nodus (node).
''Arthrorhachis'' is derived from the Greek ἄρθρον (''árthron'', "joint") and ῥάχις ( ''rháchis'', meaning axis, spine, ridge or backbone).


Taxonomy

''Trinodus'', ''Arthrorhachis'' and '' Geragnostus'' are closely related and it may be appropriate to assign their species to just one genus. All species in these three genera have virtually identical cephalons, but of ''T. agnostiformes'', the type species of ''Trinodus'' only one poorly preserved cephalon was known. However, relatively recent, pygidia assignable to ''T. agnostiformes'' were found. Although this material is distorted or incompletely preserved, it is very similar to the pygidium of ''Arthrorhachis tarda''. Species with a rear rhachis lobe longer than the postaxial region are henceforth combined in ''Geragnostus'', all others are assigned to ''Trinodus''.


Species previously assigned to ''Trinodus''

* ''T. glabratus'' var. ''kirgizica'' = '' Geragnostus kirgizica'' * ''T. kirgizica'' = '' Geragnostus kirgizica'' * ''T. longicollis'' = '' Geragnostus longicollis''


Distribution

* ''T. agnostiformes'' was identified in the Upper Ordovician of Ireland (Caradoc, Greenville slates, Campile Formation). * ''Trinodus danicus'' occurs in Ordovician of Svalbard (Ibexian 488.3 - 471.8 Mya, ''Psephosthenaspis nasuta'' trilobite zone, Olenidsletta Member, Valhallfonna Formation, Ny Friesland, 79.8° N, 17.8° E). * ''Trinodus elspethi'' is present in the Upper Ordovician of the USA (Kathian?, Athens Shale, Ina railroad cutting, South of Otes, North of Bulls Gap, Hawkins, Tennessee). * ''T. hupehensis'' has been found in the Middle Ordovician of China (
Dawan Formation The term Dawan may refer to: Places Yemen * Wadi Dawan in Hadhramaut, Yemen * Daw'an District of the Hadhramaut Governorate, Yemen Other uses * Atoni, also known as Dawan, an ethnic group on Timor * Dayuan, a historical people of West China (pi ...
, upper Arenig, Fenxiang, Yichang County, West Hubei). * ''Trinodus knockerkensis'' has been collected from the Ordovician of Ireland (Lower Caradoc, Brickwork's Quarry Shales Member, Knockerk Formation, 53.8° N, 6.6° W). * ''Trinodus pragensis'' is present in the Upper Ordovician of the Czech Republic (Lower Katian, abandoned brickyard “Na bílém koni”, Bohdalec Formation, Karlíkore Horizon, Praha-Hloubětín). ''A. pragensis'' precedes ''A. tardus'' in Czech Republic. * ''Trinodus tardus'' is present in the Upper Ordovician of Denmark (Læså - stream section at Vasegård -, Øleå - stream section at Billegrav Gård -, and Risebæk, all Bornholm), of Sweden (In
Skåne Scania ( ), also known by its native name of Skåne (), is the southernmost of the historical provinces of Sweden, provinces () of Sweden. Located in the south tip of the geographical region of Götaland, the province is roughly conterminous w ...
: Koången in the classical Fågelsång area east of Lund; Koången core; Lindegård core; Rostånga; Tommarp; Jerrestad, stream section in the Jerrestadsån rivulet; Tosterup. In
Östergötland Östergötland (; English exonym: East Gothland) is one of the traditional provinces of Sweden (''landskap'' in Swedish) in the south of Sweden. It borders Småland, Västergötland, Närke, Södermanland and the Baltic Sea. In older English li ...
: Råssnäs and Rödbergsudde, both near
Motala Motala () is a Urban areas in Sweden, locality and the seat of Motala Municipality, Östergötland County, Sweden with a municipal population of 43,717 inhabitants in 2024. It is the third largest city of Östergötland, following Linköping and N ...
. In
Västergötland Västergötland (), also known as West Gothland or the Latinized version Westrogothia in older literature, is one of the 25 traditional non-administrative provinces of Sweden (''landskap'' in Swedish), situated in the southwest of Sweden. Vä ...
: Lower Jonstorp Formation "Green ''Tretaspis'' Shale" at Bestorp – Mosseberg. Upper Jonstorp Formation "Red ''Tretaspis'' Shale" and Ulunda Mudstone of the upper Jerrestadian Stage at Stommen (Kungslena); Varvsberget, Skogastorp, Plantaberget (Högstenaberget); Skultorp, Billingen; Rustsäter, Kinnekulle; Kullatorp core, Kinnekulle. In
Dalarna Dalarna (; ), also referred to by the English exonyms Dalecarlia and the Dales, is a (historical province) in central Sweden. Dalarna adjoins Härjedalen, Hälsingland, Gästrikland, Västmanland and Värmland. It is also bordered by Nor ...
: Vikarbyn and Gulleråsen-Sanden) of Norway (Oslo Region, Hovedøya, Spannslokket Member of the Skogerholmen Formation; Åslund, Hadeland Gagnum Shale Member of the Lunner Formation) of the Czech Republic (upper Katian, ''Tretaspis seticornis'' community, Kraluv Dvur Formation), of Italy ( ''Foliomena'' community, Domusnovas Formation, Punta S'Argiola, Sardinia, 40.0° N, 8.0° E), of Iran (late Caradoc, early Asgill, Tatavrud, 35 km southwest of Bandar-e-Anzali), and of Kazakhstan. * ''Trinodus sp.'' (presumably ''T. tardus'') has been collected from the Ordovician of Czech Republic ( Králův Dvůr Formation, Prague Basin). * ''Trinodus sps.'' are found in the Ordovician of Canada (Llanvirn, Road River Formation, ''Peraspis'' fauna, Yukon Territory, 62.8° N, 136.6° W, and Cnemidopyge fauna, 65.0° N, 111.5° W), of China (Baota (Pagoda) Formation, NingLang, NingLang County, Yunnan), of Poland (Hirnantian, Holycross Mountains, 51.0° N, 21.0° E), of Wales (North Wales Rawtheyan Assemblage 4C, Crugan Mudstone and Duyfor Mudstone fauna, 53.0° N, 4.0° W; Rhwlas Limestone Formation and ''Opsimasaphus''-''Nankinolithus'' trilobite association, 53.0° N, 3.0° W; Pontyfeni Formation, ''B. rushtoni'' and ''Stapeleyella abyfrons'' biozone,
Whitland Whitland (, , or , , from the medieval ) is a town and Community (Wales), community in Carmarthenshire, Wales. Geography The Whitland Community (Wales), community is bordered by the communities of: Henllanfallteg; Llanboidy; and Eglwyscummin, a ...
, 52.0° N, 5.0° W) and Scotland (Costonian, Superstes Mudstone, Stinchar River tributary,
Colmonell Colmonell (Scottish Gaelic: ''Cill Cholmain Eala''; meaning the church of St. Colman of Lainn Eala – in Lynally, County Offaly, Ireland) is a small village and civil parish in the Stinchar Valley, South Ayrshire, Scotland. The nearest town is ...
, Gircan District, Strathclyde, 57.0° N, 4.0° W).


Development

''Trinodus elspethi'', which - as an agnostoid - only has two thorax segments, has at least nine larval stages (or instars), three meraspid and six holaspid, in its life. So it molted at least eight times. Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997


References

{{Taxonbar, from=Q4797689 Agnostida genera Agnostoidea Ordovician trilobites Fossils of Canada Fossils of the United States Fossils of Italy Fossils of the Czech Republic Fossils of Poland Fossils of Denmark Fossils of Sweden Fossils of Svalbard Fossils of Ireland Fossils of Great Britain Fossils of Iran Fossils of Kazakhstan Fossils of China Paleontology in Yukon Paleozoic life of the Northwest Territories Taxa named by Frederick McCoy Paleozoic life of Yukon