Homosteus

{{Short description|Genus of fossil fishes}} {{Automatic taxobox |fossil_range = Eifelian-?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 of flattened arthrodire placoderm from the Middle Devonian. Fossils are found primarily in Eifelian-epoch aged strata of Europe, Canada, Greenland, and Estonia.<ref name=Denison>{{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}}</ref> 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.<ref>{{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}}</ref>

''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].<ref>{{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}}</ref><ref>{{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 }}</ref><ref name="caithnessbulletin">{{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}}</ref> 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),<ref name="caithnessbulletin"></ref> rather than radioactive isotopes being absorbed by the bones during fossilization (as in most cases of radioactive fossils).<ref>{{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}}</ref> 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.<ref name="caithnessbulletin"></ref>

==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.<ref name=Denison />

===''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.<ref name=Denison />

===''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.<ref name=Denison />

===''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.<ref name=Denison />

===''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"<ref name=Denison />

===''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.<ref name=Denison />

===''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.<ref name=Denison /> It is named for Hugh Miller.<ref>{{cite web |title=Collections: Fossil - 'Homosteus milleri traq.' |url=https://www.stromnessmuseum.org.uk/collections/objects/g055 |publisher=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}}</ref>

===''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''.<ref name=Denison />

==References== {{Reflist}}

Category:Homostiidae Category:Eifelian life Category:Placoderms of Europe Category:Fossils of Estonia Category:Fossils of Norway Category:Fossils of Great Britain Category:Placoderms of North America Category:Fossils of Canada Category:Fossils of Greenland Category:Fossil taxa described in 1856