Homostiidae is a family 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 ...

placoderms from the Early to 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, whe ...

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s appear in various strata in

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Russia Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

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 tot ...

and

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 t ...

. Many homostiids have " toothless"

jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...

s, and large sizes. suggesting that many homostiids were probably

filter feeders Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

, like the also noticeably flattened

Rhincodon typus The whale shark (''Rhincodon typus'') is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Branch TA, Chen C, Cosgrove J, Do ...

. All homostiids have flattened and elongated skulls. According to Denison 1978, primitive homostiids have moderately long median dorsal plates, whereas in "advanced" homostiids, the median dorsal tends to be short and broad. Obruchev (1964) placed the following primitive genera ''Euleptaspis'', ''Lophostracon'' and ''Luetkeichthys'' in a separate family, "Euleptaspididae," and Ørvig (1969), claimed that the Euleptaspidids were totally unrelated to Homostiidae proper (i.e., being neither related to, nor ancestral), but, according to Denison, did not clearly explain his reasons why this was so.

Genera

''Angarichthys''

A comparatively large animal from the Middle Devonian of

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...

, with a head shield estimated around 40 centimeters long. Known only from an infragnathal bone, and an intero-lateral and a marginal plate.

''Antineosteus''

A primitive homostiid from

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 the ...

-aged strata of

. ''Antineosteus''' primitive anatomical features suggest it may be a precursor to ''Angarichthys, Atlantidosteus'' and ''Homosteus''. ''Antineosteus'' lived

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 sh ...

ally with the Moroccan species of ''Atlantidosteus''.

''Atlantidosteus''

This genus is known from species found in Emsian-aged Morocco and Middle Devonian Australia. Overall form is very similar to ''Antineosteus'' and ''Homosteus''.

''Cavanosteus''

A primitive genus from Emsian-aged strata of Australia

''Cathlesichthys''

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of homostiid from

Wee Jasper Wee Jasper is a hamlet in the Goodradigbee River, Goodradigbee valley at the western foot of the Brindabella Ranges, near Burrinjuck Dam in New South Wales, Australia in Yass Valley Council, Yass Valley Shire. It is located about 90 km nort ...

, living during the Early Devonian.

''Euleptaspis''

This genus is known from isolated plates and fragments from Lower Devonian-aged strata in

Spitzbergen Svalbard ( , ), also known as Spitsbergen, or Spitzbergen, is a Norwegian archipelago in the Arctic Ocean. North of mainland Europe, it is about midway between the northern coast of Norway and the North Pole. The islands of the group range ...

and

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

. The holotype of the type species, ''E. depressa'', is a paranuchal plate very similar to those seen in coccosteids.

''Dhanguura''

A very large, primitive form from the Early Devonian Taemas-Wee Jasper Reef fauna. Its discoverer, Gavin Young, hypothesizes that it may have been a

filter-feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

''Homosteus''

The type genus, known from both complete and fragmentary fossils in Europe, Russia, and North America.

''Lophostracon''

''Luetkeichthys''

''Tityosteus''

''Tityosteus'' is thought to be the largest vertebrate known from the Lower Devonian, with an estimated length of 2.5 meters. The holotype is an incomplete individual from the

Hunsrück The Hunsrück () is a long, triangular, pronounced upland in Rhineland-Palatinate, Germany. It is bounded by the valleys of the Moselle-Saar (north-to-west), the Nahe (south), and the Rhine (east). It is continued by the Taunus mountains, past ...

References

Placoderm families Middle Devonian extinctions Early Devonian first appearances {{Placoderm-stub