# Homosteus
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{{Short description|Genus of fossil fishes}}
{{Automatic taxobox
|fossil_range = [Eifelian](/source/Eifelian)-?[Givetian](/source/Givetian)
|image = Homostius2DB.jpg
|image_caption = ''H. milleri''
|taxon = Homosteus
|type_species = ''Homosteus formosissimus''
|type_species_authority = Asmuss 1856
|subdivision_ranks = Species
|subdivision =
* †''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''''' (also commonly spelled '''''Homostius''''') is a [genus](/source/genus) of flattened [arthrodire](/source/arthrodire) [placoderm](/source/placoderm) from the Middle [Devonian](/source/Devonian). Fossils are found primarily in [Eifelian](/source/Eifelian)-epoch aged strata of [Europe](/source/Europe), [Canada](/source/Canada), [Greenland](/source/Greenland), and [Estonia](/source/Estonia).[{{cite book|last=Denison|first=Robert|title=Placodermi ''Volume 2 of Handbook of Paleoichthyology'''|year=1978|publisher=Gustav Fischer Verlag|location=Stuttgart New York|isbn=978-0-89574-027-4|page=71}}] 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](/source/Titanichthys)'', in contrast, suggests that species of ''Homosteus'' may have been filter-feeders instead.[{{Cite journal|url=https://royalsocietypublishing.org/doi/10.1098/rsos.200272|doi = 10.1098/rsos.200272|doi-access = free|title = Was the Devonian placoderm Titanichthys a suspension feeder?|year = 2020|last1 = Coatham|first1 = Samuel J.|last2 = Vinther|first2 = Jakob|last3 = Rayfield|first3 = Emily J.|last4 = Klug|first4 = Christian|journal = Royal Society Open Science|volume = 7|issue = 5|article-number = 200272|pmid = 32537223|pmc = 7277245}}]
''Homosteus'' specimens from the [Old Red Sandstone](/source/Old_Red_Sandstone) of [Scotland](/source/Scotland) are known to be significantly radioactive, on the order of 1.2 * 104 gamma/min/g [sic].[{{cite journal |last1=Bowie |first1=S. H. U. |last2=Atkin |first2=D. |title=An Unusually Radioactive Fossil Fish from Thurso, Scotland |journal=Nature |date=March 1956 |volume=177 |issue=4506 |pages=487–488 |doi=10.1038/177487b0}}][{{cite journal |last1=Diggle |first1=W. R. |last2=Saxon |first2=J. |title=An Unusually Radioactive Fossil Fish from Thurso, Scotland |journal=Nature |date=October 1965 |volume=208 |issue=5008 |pages=400–400 |doi=10.1038/208400a0|doi-access=free }}][{{cite web |year=1979 |last1=Saxon |first1=J. |title=The radioactive fossil fishes of the north of Scotland |url=https://www.caithness.org/caithnessfieldclub/bulletins/1979/april/radioactivefossilfishes.htm |website=Caithness Field Club Bulletin |access-date=31 December 2022}}] Notably, ''Homosteus'' specimens are the only fish fossils from the [Old Red Sandstone](/source/Old_Red_Sandstone) to show significant [radioactivity](/source/radioactivity). This suggests that these specimens became radioactive from the animals ingesting radioactive isotopes in life (e.g., through ingesting radioactive [sediment](/source/sediment)),[{{cite book |last1=Kielan-Jaworowska |first1=Zofia |title=In pursuit of early mammals |date=2013 |publisher=Indiana University Press |location=Bloomington |isbn=978-0-253-00817-6}}] Individuals of ''Homosteus'' from the [Old Red Sandstone](/source/Old_Red_Sandstone) were chronically exposed to enough radiation that these animals would be expected to suffer negative effects of [radiation exposure](/source/radiation_exposure). However, no specimen of ''Homosteus'' shows any sign of [bone cancer](/source/osteosarcoma) or other radiation-induced pathologies.[{{cite web |title=Collections: Fossil - 'Homosteus milleri traq.' |url=https://www.stromnessmuseum.org.uk/collections/objects/g055 |publisher=[Stromness Museum](/source/Stromness_Museum) |access-date=2024-06-19|archive-url=https://web.archive.org/web/20240217154210/https://www.stromnessmuseum.org.uk/collections/objects/g055|archive-date=2024-02-17|url-status=live}}]
===''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''.