# Geragnostus
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{{Short description|Extinct genus of trilobites}}
{{Automatic taxobox
| fossil_range = {{Geological range|485|442|Ordovician ([Tremadocian](/source/Tremadocian) to [Ashgill](/source/Ashgill_(age)))}}
| 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](/source/%3Asv%3AGustaf_Linnarsson), 1869) ([type](/source/Type_(biology))) [synonym](/source/Synonym_(taxonomy)) ''Agnostus sidenbladhi''
* ''G. balanolobus'' Turvey, 2005
* ''G. clusus'' [Whittington](/source/Harry_B._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''[{{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 }}]
* ''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[Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997][{{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}}]
}}
'''''Geragnostus''''' is a genus of very small [agnostid](/source/agnostida) [trilobite](/source/trilobite)s whose fossils are found [Ordovician](/source/Ordovician)-aged marine strata from [Eurasia](/source/Eurasia), [North America](/source/North_America) and [Argentina](/source/Argentina).
== Etymology ==
* ''balanolobus'' is derived from the [Greek](/source/Greek_language) '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](/source/%3Asv%3AElis_Sidenbladh).
* ''waldorfstatleri'' is named after the [Muppet](/source/Muppet)s [Statler and Waldorf](/source/Statler_and_Waldorf) for the likeness of the rhachis to their heads.[{{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}}]
=== Species previously assigned to ''Geragnostus'' ===
* ''G. chronius'' = ''[Oncagnostus (Strictagnostus) chronius](/source/Oncagnostus)''
* ''G. hirundo'' = ''[Segmentagnostus hirundo](/source/Segmentagnostus)''
* ''G. scoltonensis'' = ''[Segmentagnostus scoltonensis](/source/Segmentagnostus)''
== Distribution ==
* '''''G. clusus''''', was found in the Middle Ordovician of Canada (Llanvirn, Lower Head, Newfoundland)[[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=176788 ''Geragnostus fabius''] at [Fossilworks](/source/Fossilworks).org]
* '''''G. fenhsiangensis''''', is known from the Middle Ordovician of China (Dawan Formation, upper Arenig, Fenxiang, Yichang County, West Hubei)[[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=176786 ''Geragnostus longicollis''] at [Fossilworks](/source/Fossilworks).org]
* '''''G. occitanus''''', is present in the Lower and Middle Ordovician of France (Arenig, Montagne Noire)[[https://paleobiodb.org/classic/displayCollResults?collection_no=8862 Guaviare River Ordovician] at [Fossilworks](/source/Fossilworks).org]
== Description ==
Like all Agnostida, ''Geragnostus'' is diminutive, with the headshield (or [cephalon](/source/Cephalon_(arthropod_head))) and tailshield (or [pygidium](/source/pygidium)) of approximately the same size (or [isopygous](/source/Pygidium)) 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](/source/Cephalon_(arthropod_head))). 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](/source/Trilobite) 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.