Hadrosteidae is a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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 ...

s from the Late

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 ...

. It was originally erected for the late

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 ...

Hadrosteus ''Hadrosteus'' is an extinct monospecific genus of large arthrodire placoderm from the Late Frasnian (Late Devonian) Kellwasserkalk facies of Bad Wildungen, Germany. It had large, double-pronged inferognathals (lower jawbones), and serrated edges ...

'', from the Kellwasserkalk facies. Later, the family was subjectively subsumed into

Dinichthyidae ''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 ...

due to ''Hadrosteus''' anatomical similarities with ''

Dunkleosteus ''Dunkleosteus'' is an extinct genus of large arthrodira, arthrodire ("jointed-neck") fish that existed during the Late Devonian period, about 382–358 million years ago. It was a pelagic fish inhabiting open waters, and one of the first verteb ...

'' and ''

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 ...

''. In 1967, Obruchev placed the enigmatic arthrodire ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' ''

Diplognathus ''Diplognathus'' is a genus of arthrodire placoderm from the Late Famennian Cleveland Shale of Late Devonian Ohio, known only from incomplete fragments of jaws and skulls. What fragments are known suggest that the living animals were large-eyed ...

'', from the Eastern American

Famennian The Famennian is the later of two faunal stages in the Late Devonian epoch. The most recent estimate for its duration is that it lasted from around 371.1 to 359.3 million years ago. An earlier 2012 estimate, still used by the International Commis ...

, within Hadrosteidae, on the basis of how the two genera have similar denticle ("teeth") patterns of the inferognathals, though, Denison (1978) contests this placement. With the redefining of Dinichthyidae as a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

family for ''Dinichthys'', and only a few other genera placed within

Dunkleosteidae Dunkleosteidae is an extinct family (biology), family of arthrodire placoderms that lived during the Devonian period. The gigantic apex predator ''Dunkleosteus terrelli'' is the best known member of this group. Phylogeny While members of Dunkleo ...

, Hadrosteidae is now seen as a valid family again.Carr, R.K. & W.J. Hlavin, 2010: Two new species of ''Dunkleosteus''

References

Arthrodires Placoderm families Late Devonian first appearances Late Devonian animals Late Devonian extinctions {{Placoderm-stub