''Holonema'' is an extinct genus of relatively large, barrel-shaped

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 that were found in oceans throughout the world from the Mid to Late

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 ...

, when the last species perished in the Frasnian-Fammian extinction event. Most species of the genus are known from fragments of their armor, but the

Gogo Reef The Gogo Formation in the Kimberley region of Western Australia is a Lagerstätte that exhibits exceptional preservation of a Devonian reef community. The formation is named after Gogo Station, a cattle station where outcrops appear and fossils ...

species, ''H. westolli'', is known from whole, articulated specimens. Holonema is thought to have been a benthic-dwelling fish, based on its cambered body shape, which performs better hydrodynamically when closer to the sea bed.https://sciencepress.mnhn.fr/sites/default/files/articles/pdf/g1999n1a4.pdf According to these specimens, species of ''Holonema'' lived by grazing on stony, horn-shaped,

stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). The ...

-like algae called oncholite, apparently by snipping off the points with a specialized snout.

Species

''H. rugosum''

''H. rugosum'' is the type species. It was originally described as a species of the

antiarch Antiarchi ("opposite anus") is an order of heavily armored placoderms. The antiarchs form the second-most successful group of placoderms after the arthrodires in terms of numbers of species and range of environments. The order's name was coi ...

genus, '' Pterichthys''. Fossils are found in

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during th ...

strata of

New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...

and

Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...

, and

Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in Fra ...

strata of

Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...

and

Michigan Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and the ...

. It is a fairly large species, though the type specimen is of an 18 centimeter long median dorsal plate belonging to a small individual. Fossils of ''H. horridum'' and ''Glyptaspis eastmani'' may be this species. Fossils from Frasnian Iran (''H. cf. rugosum'') may also belong to this species.

''H. arcticum''

A large species from Givetian-aged strata in

Novaya Zemlya Novaya Zemlya (, also , ; rus, Но́вая Земля́, p=ˈnovəjə zʲɪmˈlʲa, ) is an archipelago in northern Russia. It is situated in the Arctic Ocean, in the extreme northeast of Europe, with Cape Flissingsky, on the northern island, ...

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 ...

, known only from two plates from one individual's trunk shield.

''H. bruehni''

''H. bruehni'' is a recently discovered species from the Middle Eifelian-aged Brandenberg stratum of

Sauerland The Sauerland () is a rural, hilly area spreading across most of the south-eastern part of North Rhine-Westphalia, in parts heavily forested and, apart from the major valleys, sparsely inhabited. The Sauerland is the largest tourist region in ...

, Northwestern Germany. Juveniles had a dermal ornamentation of irregularly spaced tubercles, while the dermal surface of adults' armor were smooth.Otto, Markus. "New finds of vertebrates in the Middle Devonian Brandenberg Group (Sauerland, Northwest Germany)." Paläontologische Zeitschrift 72, no. 1-2 (1998): 117-134.

''H. farrowi''

Known from fragments from the Givetian-aged "Traverse Group" stratum in

. The median dorsal plates range up to 20 cm in length. The median dorsal plate is fairly broad at the posterior end, but narrows as one approaches the anterior end.

''H. haiti''

Known from fragments from the Givetian section of the Jefferson formation in

Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyom ...

. The median dorsal plate is estimated to be about 28 cm in length, and is very narrow compared to those of other species.

''H. harmae''

Found in the Givetian-aged Burtnieki beds of

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, a ...

. It is distinguished from other species in that the spinal and subpectoral margins of the anterior-ventral plates meet at almost a right angle.

''H. obrutshevi''

Found in the

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 s ...

-aged Aruküla beds of Estonia. The median dorsal plate has a small, horn-like process, and the trunkshield is somewhat circular when viewed from the top or bottom.

''H. ornatum''

From the Givetian-aged Bressay flags of the

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 no ...

. Imperfectly known. Denison 1978 suggests that it may not belong to ''Holonema''.

''H. radiatum''

This species is known primarily from the Frasnian-aged Shelon beds of the Baltic, and the Donetz Basin of Russia. Specimens and fragments from the Frasnian-aged Holy Cross Mountain of Poland, and the Givetian sections of the Fiskekløfta formation 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 ...

. It is a fairly large species with a moderately narrow median dorsal plate. The median dorsal plate averages about 30 cm in length.

''H. westolli''

This is the best-known species of the genus, known from numerous, often articulated specimens from the Frasnian-aged Gogo Reef. The species is very similar to ''H. rugosum'', but the trunk shield is comparatively more narrow. The median dorsal averages about 23 cm in length, and the average body length of living individuals are estimated to be around 150 cm.