Trinodus

Trinodus is a very small to small (about Template:Convert) blind trilobite, a well known group of extinct marine arthropods, which lived during the Ordovician (Tremadocian to early Hirnantian),<ref name="Budil"/> 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).<ref name="Budil"/>

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.<ref name='Turvey'>Template:Cite journal</ref>

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).<ref name='Karim'>Template:Cite journal</ref>
• 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).<ref>Template:Cite journal</ref>
• 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).<ref name="Budil"/>
• T. hupehensis has been found in the Middle Ordovician of China (Dawan Formation, upper Arenig, Fenxiang, Yichang County, West Hubei).<ref name='Turvey'/>
• 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).<ref>Template:Cite journal</ref>
• 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.<ref name="Budil"/>
• 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),<ref name='Ahlberg'>Template:Cite journal</ref> of Sweden (In Skåne: Koången in the classical Fågelsång area east of Lund; Koången core; Lindegård core; Rostånga; Tommarp; Jerrestad, stream section in the Jerrestadsån rivulet; Tosterup. In Ostergotland: Råssnås and Rodbergsudden, both near MotalaIn. In Våstergotland: 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 (Hogstenaberget); Skultorp, Billingen; Rustsåter, Kinnekulle; Kullatorp core, Kinnekulle. In Dalarna: Vikarbyn and Gulleråsen-Sanden)<ref name='Ahlberg'/> of Norway (Oslo Region, Hovedøya, Spannslokket Member of the Skogerholmen Formation; Åslund, Hadeland Gagnum Shale Member of the Lunner Formation)<ref name='Ahlberg'/> of the Czech Republic (upper Katian, Tretaspis seticornis community, Kraluv Dvur Formation),<ref>Template:Cite book</ref> of Italy (Foliomena community, Domusnovas Formation, Punta S'Argiola, Sardinia, 40.0° N, 8.0° E),<ref>Template:Cite journal</ref> of Iran (late Caradoc, early Asgill, Tatavrud, 35 km southwest of Bandar-e-Anzali),<ref name='Karim' /> and of Kazakhstan.<ref name='Ahlberg'/>
• Trinodus sp. (presumably T. tardus) has been collected from the Ordovician of Czech Republic (Králův Dvůr Formation, Prague Basin).<ref>Template:Cite journal</ref>
• 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),<ref>Template:Cite journal</ref> of China (Baota (Pagoda) Formation, NingLang, NingLang County, Yunnan),<ref>Template:Cite book</ref> of Poland (Hirnantian, Holycross Mountains, 51.0° N, 21.0° E),<ref>Template:Cite journal</ref> of Wales (North Wales Rawtheyan Assemblage 4C, Crugan Mudstone and Duyfor Mudstone fauna, 53.0° N, 4.0° W;<ref>Template:Cite journal</ref> Rhwlas Limestone Formation<ref>Template:Cite journal</ref> and Opsimasaphus-Nankinolithus trilobite association,<ref>Template:Cite journal</ref> 53.0° N, 3.0° W; Pontyfeni Formation, B. rushtoni and Stapeleyella abyfrons biozone, Whitland, 52.0° N, 5.0° W)<ref>Template:Cite journal</ref> and Scotland (Costonian, Superstes Mudstone, Stinchar River tributary, Colmonell, Gircan District, Strathclyde, 57.0° N, 4.0° W).<ref>Template:Cite journal</ref>

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.<ref name='Treatise'>Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997</ref>