Amphiaspidida is a

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

of extinct cyathaspidid heterostracan

agnatha Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...

ns whose fossils are restricted to Lower Devonian marine strata of

Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

near the

Taimyr Peninsula The Taymyr Peninsula ( ) is a peninsula in the Far North of Russia, in the Siberian Federal District, that forms the northernmost part of the mainland of Eurasia. Administratively it is part of the Krasnoyarsk Krai Federal subject of Russia. Ge ...

. Some authorities treat it as a suborder of Cyathaspidiformes,Lundgren, Mette, and Henning Blom. "Phylogenetic relationships of the cyathaspidids (Heterostraci)." GFF 135.1 (2013): 74-84. while others treat it as an

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

in its own right as "Amphiaspidiformes."Novitskaya, Larisse. Les amphiaspides (Heterostraci) du Devonien de la Siberie. Éditions du Centre national de la recherche scientifique, 1971. In life, they are thought to be benthic animals that lived most of their lives mostly buried in the sediment of a series of hypersaline lagoons. Amphiaspids are easily distinguished from other heterostracans in that all of the plates of the cephalothorax armor are fused into a single, muff-like unit, so that the forebody of the living animal would have looked like a potpie or a hot waterbottle with a pair of small, or degenerated eyes sometimes flanked by preorbital openings, a pair of branchial openings for exhaling, and a simple, slit-like, or tube-like mouth.

Taxonomy

Amphiaspidida is traditionally regarded as the sister-taxon or daughter-taxon of the cyathaspidid family Ctenaspidae (ne "Ctenaspididae"), though no formal shared traits are identified between the two groups. The ctenaspidid cyathaspid, '' Boothiaspis'', of Lower Devonian Canada, was initially described as an amphiaspid. As mentioned earlier, Amphiaspidida is treated as either a suborder of Cyathaspidiformes, or as an order in its own right, sometimes referred to as "Amphiaspidiformes." Regardless of its own status, Amphiaspidida is divided into three superfamilies, Amphiaspidoidei, Hibernaspidoidei, and Siberiaspidoidei. An additional species, '' Gunaspis orientalis'', is treated as Amphiaspidida ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' because it is known only from fragments with distinctive micro-ornamentation.

Amphiaspidoidei

The superfamily Amphiaspidoidei contains four families.

Amphiaspididae

This family contains two monotypic genera, including the type genus ''

Amphiaspis ''Amphiaspis argo'' is the type species of the cyathaspidid taxon Amphiaspidida, and of the family Amphiaspididae.Novitskaya, Larisse. Les amphiaspides (Heterostraci) du Devonien de la Siberie. Éditions du Centre national de la recherche scient ...

'', and ''

Amphoraspis ''Amphoraspis stellata'' is an amphiaspidid heterostracan in the family Amphiaspididae. Its fossils are restricted to early Devonian-aged marine strata of the Taimyr Peninsula, Siberia. ''A. stellata'', as with all other amphiaspidids, is thou ...

Edaphaspididae

This family is monotypic, and contains the monotypic genus ''

Edaphaspis ''Edaphaspis bystrowi'' is an extinct amphiaspidid cyathaspidid heterostracan.Novitskaya, Larisse. Les amphiaspides (Heterostraci) du Devonien de la Siberie. Éditions du Centre national de la recherche scientifique, 1971. Its fossils are res ...

''.

Gabreyaspididae

This family contains four genera, '' Gabreyaspis'', '' Prosarctaspis'', '' Pelaspis'', and '' Tareyaspis''.

Olbiaspididae

This family contains three genera, '' Olbiaspis'', '' Kureykaspis'' and '' Angaraspis''.

Hibernaspidoidei

This superfamily contains three families.

Hibernaspididae

This family is monotypic, containing only the genus ''

Hibernaspis ''Hibernaspis'' is a genus of extinct amphiaspidid heterostracan agnathans whose fossils are restricted to Lower Devonian marine strata of Siberia near the Taimyr Peninsula. In life, species of ''Hibernaspis'' were thought to be benthic animal ...

''.

Eglonaspididae

This is a diverse family that contains five genera, including '' Eglonaspis'', '' Gerronaspis'', '' Lecaniaspis'', '' Empedaspis'', and '' Pelurgaspis''.

Aphataspididae

This family contains two monotypic genera, '' Aphataspis'' and '' Putoranaspis''.

Siberiaspidoidei

This superfamily contains two families, Siberiaspididae, and Tuxeraspididae.

Siberiaspididae

This family contains two monotypic genera, '' Siberiaspis'', and '' Argyriaspis''.

Tuxeraspididae

This family contains three monotypic genera, '' Tuxeraspis'', '' Litotaspis'', and '' Dotaspis''. ''Tuxeraspis'' and ''Litotaspis'' are known primarily from fragments and portions of the head-region, while ''Dotaspis'' is known from a mostly intact headshield.Новицкая ЛИ (1986), "Древнейшие бесчелюстные СССР: Гетеростраки: Циaтacпиды, Aмфиacпиды, Птepacпиды." Труды ПИН 219: 159c. ovitskaya, LI (1986), The most ancient agnathans of the USSR: Heterostraci: Cyathaspidae, Amphiaspidae, Pteraspidae. Trudy Paleontol. Inst. Nauk 219: 159 pp./ref>

References

{{Taxonbar, from1=Q20311053, from2=Q35136202 Devonian jawless fish Early Devonian fish of Europe Devonian fish of Asia Fossils of Russia Fauna of Siberia Fish of Russia Endemic fauna of Russia Krasnoyarsk Krai Cyathaspidiformes Early Devonian first appearances Early Devonian extinctions