''Geragnostus'' is a genus of very small

agnostid Agnostidae is a family of Agnostida trilobites. Like all Agnostina, they were eyeless and had only two thoracic segments. These trilobites inhabited benthic waters worldwide from 508 to 461 million years ago. The family includes the following gene ...

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 ...

s whose fossils are found

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-aged marine strata from

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and

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Etymology

* ''balanolobus'' is derived from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

'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

Muppet The Muppets are an American ensemble cast of puppet characters known for an absurdist, slapstick, burlesque, and self-referential style of musical variety-sketch comedy. Created by Jim Henson in 1955, the eponymous media franchise encompasse ...

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for the likeness of the rhachis to their heads.

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.

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) * ''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) * ''G. fabius'', Canada (Newfoundland and Labrador)''Geragnostus fabius''
at

Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.org * ''G. fenhsiangensis'', is known from the Middle Ordovician of China (Dawan Formation, upper Arenig, Fenxiang, Yichang County, West Hubei) * ''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) * ''G. longicollis'', in Canada and China''Geragnostus longicollis''
at

.org * ''G. occitanus'', is present in the Lower and Middle Ordovician of France (Arenig, Montagne Noire) * ''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) * ''G. splendens'', was excavated from the Lower and Middle Ordovician of Baltica and the Czech Republic (Arenig) * ''G. waldorfstatleri'', has been found in the Lower and Middle Ordovician of South China (Arenig-Llanvirn) Unidentified species in the genus ''Geragnostus'' have among others been found in Colombia (

Guaviare River The Guaviare is a tributary of the Orinoco in Colombia. It flows together with the upper Orinoco (until here also called Río Parágua), which it clearly surpasses in length (altogether about 1,760 km) and water flow. Thus, the Guaviare is hydrol ...

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).Guaviare River Ordovician
at

.org

Description

Like all Agnostida, ''Geragnostus'' is diminutive, with the headshield (or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

) and tailshield (or

pygidium The pygidium (: pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compos ...

) 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, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior ...

). The glabella is about 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 – times as long as the pygidium, almost parallel-sided, very slightly constricted at the middle lobe (M2), broadly rounded, and – 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 – 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.

References

{{Taxonbar, from=Q1009594 Agnostoidea Agnostida genera Ordovician trilobites Ordovician trilobites of Africa Fossils of Morocco Fezouata Formation fossils Ordovician trilobites of Asia Fossils of Turkey Fossils of China Fossils of Malaysia Fossils of Iran Ordovician trilobites of Europe Fossils of Sweden Fossils of Norway Fossils of France Fossils of the Czech Republic Fossils of Great Britain Ordovician trilobites of North America Paleozoic life of Newfoundland and Labrador Paleozoic life of the Northwest Territories Fossils of Canada Fossils of the United States Ordovician trilobites of South America Ordovician Argentina Fossils of Argentina Ordovician Colombia Fossils of Colombia Fossil taxa described in 1935