Cyathaspidida is a

taxon In biology, a taxon (back-formation from ''taxonomy''; plural 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 nam ...

of extinct cyathaspidiform heterostracan agnathans whose fossils are found in Silurian to Lower Devonian marine strata of

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

and

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.Lundgren, Mette, and Henning Blom. "Phylogenetic relationships of the cyathaspidids (Heterostraci)." GFF 135.1 (2013): 74-84. In life, they are thought to be benthic animals that lived most of their lives either mostly buried in or resting directly on top of the substrate.

Taxonomy

The cyathaspidids of Cyathaspidida were tadpole-like animals with drum-shaped, cigar-shaped or wedge-shaped cephalothoraxes, and were anatomically similar to several other heterostracan groups. However, with some groups, such as the traquairaspids, cardiopeltids, and corvaspidids, this similarity appears to be superficial. With other groups, namely the tolypelepidids, the similarity suggests a close relationship. The type genus of the tolypelepidids, '' Tolypelepis'', in particular, was determined to be the sister-taxon of Cyathaspidida. Originally, the cyathaspidid genera were organized together within the family Cyathaspididae. Later, the genera were then organized into subfamilies, and these subfamilies would eventually be promoted to full familial status, including Irregularaspididae, Anglaspididae, Poraspididae, Ctenaspididae, etc. In 2013, Lundgren and Blom reassessed several specimens of cyathaspidids, and reorganized them into three families, Cyathaspidae, Ariaspidae and Ctenaspidae.

Cyathaspidae

Cyathaspidae contains most of the genera originally contained within Cyathaspididae, as well as those genera contained within Irregularaspididae, and Poraspididae. In addition to the type genus, ''

Cyathaspis ''Cyathaspis'' is the type genus of the heterostracan order Cyathaspidiformes. Fossils are found in late Silurian strata in the Cunningham Creek Formation, New Brunswick, Canada and Europe, especially in the Downton Castle Sandstone of Great B ...

'', Cyathaspidae contains the following genera: '' Americaspis'', '' Archegonaspis'', ''

Capitaspis ''Capitaspis'' is an extinct genus of cyathaspidine cyathaspidid heterostracan jawless vertebrate known from the Late Silurian Somerset Island Formation ( Pridolian epoch) of the Northwest Territories, Canada. It contains a single species ...

'', '' Dikenaspis'', '' Dinaspidella'', '' Homaspidella'', '' Irregulareaspis'', '' Nahanniaspis'', '' Pionaspis'', ''

Poraspis ''Poraspis'' is an extinct genus of heterostracan. Fossils are found in Late Silurian and Early Devonian marine strata of Norway, Canada and the United States The United States of America (U.S.A. or USA), commonly known as the United ...

'', '' Ptomaspis'', '' Seretaspis'', '' Steinaspis'', '' Torpedaspis'', and '' Veronaspis''.

Ariaspidae

Family Ariaspidae contains '' Ariaspis'', and its sister-taxa originally contained within Anglaspididae/Anglaspidinae, including ''

Anglaspis ''Anglaspis'' is an extinct genus of cyathaspidiform heterostracan agnathan. Fossils are found in marine strata of Europe, from the late Silurian period until the genus' extinction during the Early Devonian. As with other cyathaspidiforms, i ...

'', '' Listraspis'', ''

Liliaspis ''Liliaspis philippovae'' is an extinct cyathaspidiform heterostracan agnathan from early Devonian marine strata of the Ural Mountains. ''Liliaspis'' was, at various times, placed in the families Cyathaspididae, Poraspididae, and Anglaspididae ...

'', and ''

Paraliliaspis ''Paraliliaspis egregia'' is an extinct cyathaspidiform heterostracan agnathan from early Devonian marine strata of the Ural Mountains. ''Paraliliaspis'' was, at various times, placed in the families Cyathaspididae, and Anglaspididae. Current ...

''.

Ctenaspidae

Amphiaspidida Amphiaspidida is a taxon of extinct cyathaspidid heterostracan agnathans whose fossils are restricted to Lower Devonian marine strata of Siberia near the Taimyr Peninsula. Some authorities treat it as a suborder of Cyathaspidiformes,Lundgre ...

can be ignored as a daughter-taxon, the family Ctenaspidae contains ''

Ctenaspis ''Ctenaspis'' (''Ctenaspis kaieri'') is an extinct genus of heterostracan cyathaspid agnathans from the early Devonian of Canada, Norway and Russia 416.0 to 412.3 million years ago. Species of ''Ctenaspis'' are typically 10 to 18 cm long with t ...

'' and its various sister-taxa originally contained within both Ctenaspididae and Ctenaspididae, including '' Allocryptaspis'', ''

Alainaspis ''Alainaspis platyrhina'' is an extinct cyathaspidid heterostracan agnathan vertebrate which existed in a marine environment in what is now the Northwest Territories of Canada, during the upper Silurian period. It was first named by David Ellio ...

'', '' Zaphoctenaspis'', '' Arctictenaspis'', and '' Boothiaspis'', which was first described as a "Canadian amphiaspid." Ctenaspis NT small

References

Devonian jawless fish Silurian jawless fish Devonian fish of Europe Silurian fish of Europe Devonian fish of North America Silurian fish of North America Cyathaspidiformes Devonian first appearances Silurian extinctions {{Devonian-jawless-fish-stub