''Homosteus'' (also commonly spelled ''Homostius'') is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of flattened

arthrodire Arthrodira (Greek for "jointed neck") is an Order (biology), 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 penetratin ...

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

from the Middle

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

. 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 continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...

, and

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

. 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 Old Red Sandstone, abbreviated ORS, 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 eastern seaboard of North America. It ...

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

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 Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

. This suggests that these specimens became radioactive from the animals ingesting radioactive isotopes in life (e.g., through ingesting radioactive

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

), 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 exposure is a measure of the ionization of air due to ionizing radiation from photons. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. As of 2007, "medical radia ...

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

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

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 () 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. Gallery File ...

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 Trionychidae is a family (biology), family of turtles, Common name, commonly known as softshell turtles or simply softshells. The family was described by Leopold Fitzinger in 1826. Softshells include some of the world's largest freshwater turtles ...

). ''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 countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

. 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 410.62 ±1.95 million years ago to 393.47 ±0.99 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 Canning is a method of food preservation in which food is processed and sealed in an airtight container ( jars like Mason jars, and steel and tin cans). Canning provides a shelf life that typically ranges from one to five years, although unde ...

, 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 closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...

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 ''Dinichthys'' (from , 'terrible' and 'fish') is an extinct monospecific genus of large marine arthrodire placoderm from the Late Devonian (Famennian stage) measuring around long. Fossils were recovered from the Ohio Shale Formation along ...

''. ''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 (), also known as the Orkney Islands, is an archipelago off the north coast of mainland Scotland. The plural name the Orkneys is also sometimes used, but locals now consider it outdated. Part of the Northern Isles along with Shetland, ...

and

Shetland Islands Shetland (until 1975 spelled Zetland), also called the Shetland Islands, is an archipelago in Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the Uni ...

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

Hugh Miller Hugh Miller (10 October 1802 – 23/24 December 1856) was a Scottish geologist, writer and folklorist. Life and work Miller was born in Cromarty, the first of three children of Harriet Wright (''bap''. 1780, ''d''. 1863) and Hugh Miller ...

''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