Geragnostus

{{Short description|Extinct genus of trilobites}} {{Automatic taxobox | fossil_range = {{Geological range|485|442|Ordovician (Tremadocian to Ashgill)}} | image = Geragnostus mediterraneus CRF.jpg | image_caption = ''Geragnostus mediterraneus'', 6mm, St.Chinian Formation, France, Middle Ordovician | taxon = Geragnostus | authority = Howell, 1935 | synonyms = ''Geragnostella'', ''Geratrinodus'', ''Neptunagnostella'' | subdivision_ranks = Species | subdivision = * ''G. sidenbladhi'' (Linnarsson, 1869) (type) synonym ''Agnostus sidenbladhi'' * ''G. balanolobus'' Turvey, 2005 * ''G. clusus'' Whittington, 1963 * ''G. consors'' (Holub, 1912) synonyms ''Agnostus consors'', ''Neptunagnostella consors'' * ''G. crassus'' Tjernvik, 1956 * ''G. fabius'' Billings, 1865 * ''G. fenhsiangensis'' Lu, 1975 * ''G. gilcidae'' Rábano, Pek & Vaněk, 1985 * ''G. hispanicus'' Rábano, Pek & Vaněk, 1985 * ''G. kirgizica'' (Weber, 1948) synonyms ''Trinodus glabratus'' var. ''kirgizica'', ''T. kirgizica''<ref name='Karim'>{{cite journal|last= Karim|first=T.S.|year= 2009|title= Ordovician trilobites from Iran|journal= Earth and Environmental Science Transactions of the Royal Society of Edinburgh|issue= 99|volume= 02|pages= 101–124|doi=10.1017/s1755691009007063 |s2cid=129780250 }}</ref> * ''G. longicollis'' Raymond, 1925 * ''G. occitanus'' Howell, 1935 * ''G. perconvexus'' (Kobayashi & Hamada, 1978) synonym ''Geratrinodus perconvexus'' * ''G. splendens'' (Holub, 1912) * ''G. tullbergi'' (Novák, 1883) synonyms ''Agnostus tullbergi'', ''Geragnostella tullbergi'' * ''G. waldorfstatleri'' Turvey, 2005<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><ref name='Turvey'>{{cite journal|last= Turvey|first= S.T.|year= 2005|title= Agnostid trilobites from the Arenig-Llanvirn of South China|journal= Transactions of the Royal Society of Edinburgh: Earth Sciences|volume= 95|issue= 3–4|pages= 527–542|doi=10.1017/s026359330000119x|s2cid= 130775617}}</ref> }}

'''''Geragnostus''''' is a genus of very small agnostid trilobites whose fossils are found Ordovician-aged marine strata from Eurasia, North America and Argentina.

== Etymology == * ''balanolobus'' is derived from the Greek 'balanos', acorn, and 'lobos', lobe, in reference to the outline of the rear lobe of the rhachis. * ''sidenbladhi'' has been named in honor of the Swedish philosopher and paleontologist Elis Sidenbladh. * ''waldorfstatleri'' is named after the Muppets Statler and Waldorf for the likeness of the rhachis to their heads.<ref name='Turvey'/>

== Taxonomy == ''Geragnostus'' is very similar to ''Trinodus'' and may in future be included in that genus. In ''Trinodus'' the posterior lobe (M3) of the pygidial axis (or rhachis) is clearly shorter than the anterior and middle lobes (M1+M2) combined, while in ''Geragnostus'' it is equal or greater in length. ''Geragnostus'' is not closely related to ''Micragnostus'', because its glabella is structurally different.<ref name='Ahlberg'>{{cite journal|last= Ahlberg|first= P.|year= 1989|title= The type species of the Ordovician agnostid trilobite ''Geragnostus'', Howell, 1935|journal= Paläontologische Zeitschrift|volume= 63|issue= 3–4|pages= 309–317|doi=10.1007/bf02989517|bibcode= 1989PalZ...63..309A|s2cid= 85408131}}</ref>

=== Species previously assigned to ''Geragnostus'' === * ''G. chronius'' = ''Oncagnostus (Strictagnostus) chronius'' * ''G. hirundo'' = ''Segmentagnostus hirundo'' * ''G. scoltonensis'' = ''Segmentagnostus scoltonensis''

== Distribution == * '''''G. clusus''''', was found in the Middle Ordovician of Canada (Llanvirn, Lower Head, Newfoundland)<ref name='Turvey'/> * '''''G. crassus''''', has been identified from the Lower Ordovician of Baltica (Tremadoc), and the Lower (Tremadoc of Yingou, Yumen County, Gansu) and Middle Ordovician of China (lower Llanvirn, Angzhanggou, Yumen County, Gansu)<ref name='Turvey'/> * '''''G. fabius''''', Canada (Newfoundland and Labrador)<ref name=FWGeragnostusFabius>[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=176788 ''Geragnostus fabius''] at Fossilworks.org</ref> * '''''G. fenhsiangensis''''', is known from the Middle Ordovician of China (Dawan Formation, upper Arenig, Fenxiang, Yichang County, West Hubei)<ref name='Turvey'/> * '''''G. gilcidae''''' and '''''G. hispanicus''''', were found in the Middle Ordovician of Spain (lower Llanvirn, Southern Central-Iberian Zone, Mounts of Toledo, Villuercas and Almadén district) * '''''G. kirgizica''''', is present in the Middle Ordovician of Kazakhstan (Karakansky horizon), and the Upper Ordovician of Iran (late Caradoc, early Asgill, Tatavrud, 35 km southwest of Bandar-e-Anzali)<ref name='Karim'/> * '''''G. longicollis''''', in Canada and China<ref name=FWGeragnostusLongicollis>[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=176786 ''Geragnostus longicollis''] at Fossilworks.org</ref> * '''''G. occitanus''''', is present in the Lower and Middle Ordovician of France (Arenig, Montagne Noire)<ref name='Turvey'/> * '''''G. sidenbladhi''''', occurs in the Lower Ordovician (upper Tremadoc, ''Ceratopyge''-limestone, ''Apatokephalus serratus''-biozone) of Sweden (Mossebo, Hunneberg, Västergötland; Fågelsång, Scania) and Norway (Trefoldighetskirken, central Oslo; Bjerkåsholmen, Slemmestad in the Oslo-Asker district; Fure at Tyrifjorden in the Modum district; and Samsal in the Ringsaker district)<ref name='Ahlberg'/> * '''''G. splendens''''', was excavated from the Lower and Middle Ordovician of Baltica and the Czech Republic (Arenig)<ref name='Turvey'/> * '''''G. waldorfstatleri''''', has been found in the Lower and Middle Ordovician of South China (Arenig-Llanvirn)<ref name='Turvey'/>

Unidentified species in the genus ''Geragnostus'' have among others been found in Colombia (Guaviare River, Meta).<ref name=FWGuaviare>[https://paleobiodb.org/classic/displayCollResults?collection_no=8862 Guaviare River Ordovician] at Fossilworks.org</ref>

== Description ==  Like all Agnostida, ''Geragnostus'' is diminutive, with the headshield (or cephalon) and tailshield (or pygidium) of approximately the same size (or isopygous) and outline. Like all Agnostina, ''Geragnostus'' has only two thorax segments, where known. The cephalon is subquadrate in outline, and almost as long as wide. The highest point is the node in the centre of the centrally raised section of the cephalon (or glabella). The glabella is about {{frac|2|3}} times the length of the cephalon, and its sides are very gently tapered forward, with a broadly rounded front, and is well defined by shallow furrows and an abrupt change in exoskeletal slope. It is 2 times as long as wide. The elongated glabellar node is prominent. The rear of the glabella is straight. Just in front of the node, a depression curves forward- and outward. The basal lobes are wider than they are long and almost touch. The border is relatively narrow, gently convex, and defined by a wide, shallow border furrow, widest at the front. The genal angles are pointed without spines.

The pygidial axis (or rhachis) is {{frac|1|2}}–{{frac|2|3}} times as long as the pygidium, almost parallel-sided, very slightly constricted at the middle lobe (M2), broadly rounded, and {{frac|1|1|3}}–{{frac|1|2|3}} times as long as wide. It has three distinct pairs of lobes, and is defined by shallow furrows. The forward lobe of the rhachis (M1) is a bit shorter and wider than the middle lobe (M2) and separated from it by a furrow (F1) which is directed outward and slightly backward from the dorsal furrow, and then curved strongly forward adaxially. The rear lobe (M3) is {{frac|1|1|3}}–{{frac|1|2|3}} longer than both others together, and the furrow is directed outward and slightly forward from the dorsal furrow. The middle lobe has a prominent tubercle, which extends backwards above the anterior portion of rear lobe. It is the highest point of the pygidium. The border is moderately wide, gently convex, defined by a shallow furrow, and widest at the rear corners which carry a pair of strong, backwardly directed spines.<ref name='Ahlberg'/>

== References == {{Portal|Paleontology}} {{Reflist}}

Category:Agnostoidea Category:Agnostida genera Category:Ordovician trilobites Category:Ordovician trilobites of Africa Category:Fossils of Morocco Category:Fezouata Formation fossils Category:Ordovician trilobites of Asia Category:Fossils of Turkey Category:Fossils of China Category:Fossils of Malaysia Category:Fossils of Iran Category:Ordovician trilobites of Europe Category:Fossils of Sweden Category:Fossils of Norway Category:Fossils of France Category:Fossils of the Czech Republic Category:Fossils of Great Britain Category:Ordovician trilobites of North America Category:Paleozoic life of Newfoundland and Labrador Category:Paleozoic life of the Northwest Territories Category:Fossils of Canada Category:Fossils of the United States Category:Ordovician trilobites of South America Category:Ordovician Argentina Category:Fossils of Argentina Category:Ordovician Colombia Category:Fossils of Colombia Category:Fossil taxa described in 1935