Zenaspidida

Zenaspidida
	Fossil of Zenaspis powriei
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	†Osteostraci
Order:	†Zenaspidida; Stensiö, 1958
Subgroups
	†Spangenhelmaspis; †Wladysagitta; †Parameteoraspididae; †Superciliaspididae; †Scolenaspididae; †Zenaspididae;
Synonyms
	Zenaspida

Zenaspidida is an extinct order of osteostracans, a group of jawless stem-gnathostomes.^[1] They possessed a distinct headshield, which varied in width to length ratio by species.

Description

The head shield is dome-shaped and extremely large in comparison to the main body. The abdominal section of this shield has a less developed median dorsal crest. As a rule for this order, the nasohypophysial opening is larger than the nasal division. The pineal plate seen in other osteostracans is barely developed or completely absent. The median dorsal field is notably broad, and the lateral fields are widened in the posterior, but reach back no further than the proximal section of the dorsal surface of the cornual processes. The ornamentation on the head shield can have singular, large tubercles, or groups of tubercles which range in size. This is often used to speciate.^[2]^[3]

Classification

The cladogram below is adapted from a 2014 article by Scott and Wilson:^[4]

Zenaspidida

	Escuminaspis

	Levesquaspis

Wladysagitta

Superciliaspididae

Glabrapelta

	Dentapelta

	Superciliaspis

Scolenaspididae

	Machairaspis

	Scolenaspis

	Ukrainaspis

Zenaspididae

Tegaspis

Stensiopelta

	Diademaspis

	Zenaspis

References

^ Scott, B. R.; Wilson, M. V. H. (2013). "A new species of osteostracan from the Lochkovian (Early Devonian) of the Mackenzie Mountains, with comments on body size, growth, and geographic distribution in the genus Machairaspis". Canadian Journal of Earth Sciences. 50 (2): 127–134. doi:10.1139/cjes-2012-0100.
^ Voichyshyn, V. (2006). "New osteostracans from the Lower Devonian terrigenousdeposits of Podolia, Ukraine". Acta Palaeontologica Polonica. 51: 131–142 – via ResearchGate.
^ Voichyshyn, V. (2011). "THE EARLY DEVONIAN ARMOURED AGNATHANS OF PODOLIA, UKRAINE" (PDF). Palaeontologia Polonica. 66.
^ Scott, B. R.; Wilson, M. V. H. (2015). "The Superciliaspididae, a new family of Early Devonian Osteostraci (jawless vertebrates) from northern Canada, with two new genera and three new species". Journal of Systematic Palaeontology. 13 (3): 167–187. doi:10.1080/14772019.2013.863809.

External links

"Zenaspidida". The Encyclopedia of Life.

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[1] Scott, B. R.; Wilson, M. V. H. (2013). "A new species of osteostracan from the Lochkovian (Early Devonian) of the Mackenzie Mountains, with comments on body size, growth, and geographic distribution in the genus Machairaspis". Canadian Journal of Earth Sciences. 50 (2): 127–134. doi:10.1139/cjes-2012-0100.

[2] Voichyshyn, V. (2006). "New osteostracans from the Lower Devonian terrigenousdeposits of Podolia, Ukraine". Acta Palaeontologica Polonica. 51: 131–142 – via ResearchGate.

[3] Voichyshyn, V. (2011). "THE EARLY DEVONIAN ARMOURED AGNATHANS OF PODOLIA, UKRAINE" (PDF). Palaeontologia Polonica. 66.

[Scott2015-4] Scott, B. R.; Wilson, M. V. H. (2015). "The Superciliaspididae, a new family of Early Devonian Osteostraci (jawless vertebrates) from northern Canada, with two new genera and three new species". Journal of Systematic Palaeontology. 13 (3): 167–187. doi:10.1080/14772019.2013.863809.

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