''Homosteus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

of flattened

arthrodire Arthrodira (Greek for "jointed neck") is an order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine e ...

placoderm from the Middle

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

. Fossils are found primarily in

Eifelian The Eifelian is the first of two faunal stages in the Middle Devonian Epoch. It lasted from 393.3 ± 1.2 million years ago to 387.7 ± 0.8 million years ago. It was preceded by the Emsian Stage and followed by the Givetian Stage. North American ...

-epoch aged 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 subcontinent of Eurasia and it is located enti ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tota ...

Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is ...

, and

Estonia Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, and t ...

. 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. ''Homosteus'' specimens from the

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also ext ...

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to th ...

are known to be significantly radioactive, on the order of 1.2 * 10⁴ gamma/min/g ic Notably, ''Homosteus'' specimens are the only fish fossils from the

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 Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

), rather than radioactive isotopes being absorbed by the bones during fossilization (as in most cases of radioactive fossils). Individuals of ''Homosteus'' from the

were chronically exposed to enough radiation that these animals would be expected to suffer negative effects of

radiation exposure Radiation is a moving form of energy, classified into ionizing and non-ionizing type. Ionizing radiation is further categorized into electromagnetic radiation (without matter) and particulate radiation (with matter). Electromagnetic radiation con ...

. However, no specimen of ''Homosteus'' shows any sign of

bone cancer A bone tumor is an abnormal growth of tissue in bone, traditionally classified as noncancerous (benign) or cancerous (malignant). Cancerous bone tumors usually originate from a cancer in another part of the body such as from lung, breast, th ...

or other radiation-induced pathologies.

Species

''H. formosissimus''

The type species of the genus. It is a thin-plated species from the Eifelian-aged

Aruküla Aruküla is a small borough ( et, alevik) in Harju County, northern Estonia. It is the administrative centre of Raasiku Parish. Aruküla had a population of 2,113 on 1 January 2020. Aruküla has a station on the Elron's eastern route. Galle ...

beds of

. 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 The Trionychidae are a taxonomic family of a number of turtle genera, commonly known as softshell turtles. The family was erected by Leopold Fitzinger in 1826. Softshells include some of the world's largest freshwater turtles, though many can a ...

). ''H. formosissimus'' had a small, thin keel down the dorsal-center of its median dorsal plate.

''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 Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...

. 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.

''H. cf. arcticus''

Based on a specimen found in the

Emsian The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after ...

epoch-aged layers of the Wood Bay formations of Spitzbergen. May or may not be of this genus.

''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.

''H. latus''

A giant species from the Eifelian-aged Aruküla beds of Estonia, and may have existed

sympatric In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species s ...

ally 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"

''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.

''H. milleri''

''H. milleri'' is from the Givetian of what is now the

Orkney Orkney (; sco, Orkney; on, Orkneyjar; nrn, Orknøjar), also known as the Orkney Islands, is an archipelago in the Northern Isles of Scotland, situated off the north coast of the island of Great Britain. Orkney is 10 miles (16 km) nort ...

and

Shetland Islands Shetland, also called the Shetland Islands and formerly Zetland, is a subarctic archipelago in Scotland lying between Orkney, the Faroe Islands and Norway. It is the northernmost region of the United Kingdom. The islands lie about to the n ...

. It is a medium-sized species with a comparatively very rectangular-shaped median dorsal plate.

''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''.

References

{{Taxonbar, from=Q16983279 Homostiidae Eifelian life Placoderms of Europe Fossils of Estonia Fossils of Norway Fossils of Great Britain Placoderms of North America Fossils of Canada Fossils of Greenland Fossil taxa described in 1856