''Aponileus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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. Chung-Hung Hu

circumscribed In geometry, a circumscribed circle for a set of points is a circle passing through each of them. Such a circle is said to ''circumscribe'' the points or a polygon formed from them; such a polygon is said to be ''inscribed'' in the circle. * Circum ...

the genus in 1963. The genus was once considered a junior subjective synonym of the genus '' Psephosthenaspis'' but it is considered to be a distinct genus again. , fossils have been found in Greenland, Texas, and Utah. They all date to the Upper

Floian The Floian is the second stage of the Ordovician Period. It succeeds the Tremadocian with which it forms the Lower Ordovician series. It precedes the Dapingian Stage of the Middle Ordovician. The Floian extended from to million years ago. The ...

(Blackhillsian) within 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 ...

Period.

Taxonomic history

''Aponileus'' was initially

by Chung-Hung Hu in 1963. The

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

is ''A. latus'', also described by Hu in 1963. The type material was poorly preserved. Hu initially said that ''Aponileus'' is "most like '' Nileus''", i.e., treating it as a nileid. Subsequently, this genus received "virtually no mention" in the trilobite literature for many years, aside from C. P. Hughes's 1979 assessment that two specimens which Lehi F. Hintze had tentatively assigned to '' Barrandia'' in 1953 should be placed in ''Aponileus''. In 2003, Jonathan M. Adrain synonymized ''Aponileus'' with '' Psephosthenaspis'', placing it in the family Bathyuridae. In 2012, Adrain and Neo E. B. McAdams treated ''Aponileus'' as a distinct genus within Bathyuridae. They also included the species ''A. glaber'', which Christian Poulsen had initially described in the genus '' Bolbocephalus'' in 1927. Adrain and McAdams placed five newly described species in ''Aponilus'', namely ''A. aasei'', ''A. belkaae'', ''A. laikaae'', ''A. strelkaae'', and ''A. ugolekae''; they also tentatively assigned a further newly described species to this genus: ''A.''? ''veterokae''. While they believed the "best supported current hypotheses" was to place it in ''Aponileus'', they acknowledged it is "very different from all other species included in the genus" and hence "assign dthe species only with question". Various

specific epithet In Taxonomy (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...

s are named after

Soviet space dogs During the 1950s and 1960s the Soviet space program used dogs for sub-orbital and Orbital spaceflight, orbital space flights to determine whether human spaceflight was feasible. The Soviet space program typically used female dogs due to the ...

Laika Laika ( ; , ; – 3 November 1957) was a Soviet space dog who was one of the first animals in space and the first to orbit the Earth. A stray mongrel from the streets of Moscow, she flew aboard the Sputnik 2 spacecraft, launched into lo ...

, Belka, Strelka, Ugolyok, and Veterok.

Phylogeny

In 2012, Adrain and McAdams said they believed that ''Aponileus'' was likely related to the "''Acidiphorus'' group", but said that the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

remained unclear. While ''Psephosthenaspis'' resembles ''Aponileus'', Adrain and McAdams believe they belong to distinct

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

s. ''Aponileus laikaae'' is the basal species.

Distribution

Chronologically, ''Aponileus'' is found entirely within the

Lower Ordovician The Early Ordovician is the first epoch of the Ordovician period, corresponding to the Lower Ordovician series of the Ordovician system. It began after the Age 10 of the Furongian epoch of the Cambrian and lasted from to million years ago, unt ...

, specifically the upper

(Blackhillsian). The genus's type species, ''A. latus'', has found in the El Paso Formation within the Franklin Mountains of west Texas. ''A. glaber'' was found in

Washington Land Washington Land is a peninsula in northwestern Greenland. It is a part of the Avannaata municipality. Washington Land was explored in the 19th century. Traditionally it was one of the hunting grounds of the Inuit, but there are archaeological rem ...

, in northwestern Greenland. The remaining species, which Adrain and McAdams first described in 2012, are all from the Confusion Range in

Millard County, Utah Millard County ( ) is a county in the U.S. state of Utah. As of the 2020 United States census, the population was 12,975. Its county seat is Fillmore, and the largest city is Delta. History The Utah Territory legislature created the county ...

Species

, the following species are recognized: * '' A. latus'' — Texas * '' A. aasei'' — Utah * '' A. glaber'' — Greenland * '' A. laikaae'' — Utah * '' A. belkaae'' — Utah * '' A. strelkaae'' — Utah * '' A. ugolekae'' — Utah * ''A.''? ''veterokae'' — Utah

References

Works cited