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Homosteus; Temporal range: Eifelian-?Givetian
	H. milleri
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	†Placodermi
Order:	†Arthrodira
Suborder:	†Brachythoraci
Family:	†Homostiidae
Genus:	†Homosteus
Type species
	Homosteus formosissimus; Asmuss 1856
Species
	†H. formosissimus Asmuss 1856; †H. arcticus Heintz 1934; †H. cf. arcticus Orvig 1969; †H. kochi Stensiö & Säve-Söderbergh 1938; †H. latus Asmuss 1856; †H. manitobensis (Okulitch) 1944; †H. milleri Traquair 1888; †H. sulcatus (Kutorga) 1837;
Synonyms
	Dinichthys manitobensis Okulitch 1944; Homostius sp. Asmuss 1856; Homosteus anceps Asmuss 1856; Homosteus cataphractus Asmuss 1856; Homosteus ponderosus Asmuss 1856; Trionyx sulcatus Kutorga 1837; Trionyx latus Kutorga 1837;

Homosteus is a genus of flattened arthrodire placoderm from the Middle Devonian. Fossils are found primarily in Eifelian-epoch aged strata of Europe, Canada, Greenland, and Estonia.^[1] All of the species had comparatively large, flattened heads with, as suggested by the upward opening orbits, upward-pointing eyes. These adaptations suggest that the various species were benthic predators. A study on Titanichthys, in contrast, suggests that species of Homosteus may have been filter-feeders instead.^[2]

Homosteus specimens from the Old Red Sandstone of Scotland are known to be significantly radioactive, on the order of 1.2 * 10⁴ gamma/min/g [sic].^[3]^[4]^[5] Notably, Homosteus specimens are the only fish fossils from the Old Red Sandstone to show significant radioactivity. This suggests that these specimens became radioactive from the animals ingesting radioactive isotopes in life (e.g., through ingesting radioactive sediment),^[5] rather than radioactive isotopes being absorbed by the bones during fossilization (as in most cases of radioactive fossils).^[6] Individuals of Homosteus from the Old Red Sandstone were chronically exposed to enough radiation that these animals would be expected to suffer negative effects of radiation exposure. However, no specimen of Homosteus shows any sign of bone cancer or other radiation-induced pathologies.^[5]

Species

H. formosissimus

The type species of the genus. It is a thin-plated species from the Eifelian-aged Aruküla beds of Estonia. Although H. sulcatus was described earlier in 1837, H. formosissimus is the official type species as it was the first to be described as a placoderm (H. sulcatus was originally described as a soft-shelled turtle). H. formosissimus had a small, thin keel down the dorsal-center of its median dorsal plate.^[1]

H. arcticus

This species is based on a 15 centimeter-long preorbital plate from the early Eifelian of the Wood Bay formations of Spitzbergen, Norway. Compared to other species, the anatomy of the plate suggests the species is very primitive for the genus. Denison, 1978, suggests that the species may be different enough to merit its own distinct genus.^[1]

H. cf. arcticus

Based on a specimen found in the Emsian epoch-aged layers of the Wood Bay formations of Spitzbergen. May or may not be of this genus.^[1]

H. kochi

This species is from the Givetian of Middle Devonian Greenland, of Canning Land, to be precise. H. kochi has a comparatively very narrow nuchal plate.^[1]

H. latus

A giant species from the Eifelian-aged Aruküla beds of Estonia, and may have existed sympatrically with H. formosissimus. H. latus differed from H. formosissimus in having comparatively thick plates, a large, massive crest-like keel along the dorsal-center of its medial dorsal plate, and head-plates over a meter in length. Originally described as "Trionyx latus Kutorga 1837"^[1]

H. manitobensis

This species is found in the Eifelian-aged Elm Point Limestone of Manitoba. Based on a pair of paranuchal and marginal plates originally referred to the genus Dinichthys. H. manitobensis is the only member of the genus found in North America proper.^[1]

H. milleri

H. milleri is from the Givetian of what is now the Orkney and Shetland Islands. It is a medium-sized species with a comparatively very rectangular-shaped median dorsal plate.^[1] It is named for Hugh Miller.^[7]

H. sulcatus

Another species from the Eifelian-aged Aruküla beds of Estonia. Although H. sulcatus was described before H. formosissimus, H. sulcatus was originally described as a soft-shelled turtle, ne "Trionyx sulcatus Kutorga 1837". H. sulcatus had thick plates, and a well-developed keel on the dorsal-center of its median dorsal plate. It was larger than H. formosissimus, but still much smaller than H. latus.^[1]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Denison, Robert (1978). Placodermi Volume 2 of Handbook of Paleoichthyology'. Stuttgart New York: Gustav Fischer Verlag. p. 71. ISBN 978-0-89574-027-4.
^ Coatham, Samuel J.; Vinther, Jakob; Rayfield, Emily J.; Klug, Christian (2020). "Was the Devonian placoderm Titanichthys a suspension feeder?". Royal Society Open Science. 7 (5): 200272. doi:10.1098/rsos.200272. PMC 7277245. PMID 32537223.
^ Bowie, S. H. U.; Atkin, D. (March 1956). "An Unusually Radioactive Fossil Fish from Thurso, Scotland". Nature. 177 (4506): 487–488. doi:10.1038/177487b0.
^ Diggle, W. R.; Saxon, J. (October 1965). "An Unusually Radioactive Fossil Fish from Thurso, Scotland". Nature. 208 (5008): 400–400. doi:10.1038/208400a0.
^ ^a ^b ^c Saxon, J. (1979). "The radioactive fossil fishes of the north of Scotland". Caithness Field Club Bulletin. Retrieved 31 December 2022.
^ Kielan-Jaworowska, Zofia (2013). In pursuit of early mammals. Bloomington: Indiana University Press. ISBN 978-0253008176.
^ "Collections: Fossil - 'Homosteus milleri traq.'". Stromness Museum. Archived from the original on 2024-02-17. Retrieved 2024-06-19.