''Ischnacanthus'' is an extinct genus of jawed fish in the class

Acanthodii Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, whic ...

. It lived during the

Early Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, whi ...

period, approximately 421–409

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...

. It is known from a single species, which is also the type species, ''I. gracilis''.

Discovery and naming

''Ischnacanthus'' was first discovered in 1861 by Egerton, and later assigned to Isnacanthidae by A. S. Woodward in 1891. The type specimen that defines this species is named ''Ischnacanthus gracilis'', and helps outline the Ischnacanthidae family it belongs to. Fossils of this fish were first discovered in Tillywhandland Quarry,

Forfar Forfar ( sco, Farfar, gd, Baile Fharfair) is the county town of Angus, Scotland and the administrative centre for Angus Council, with a new multi-million pound office complex located on the outskirts of the town. As of 2021, the town has a po ...

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 ...

. The

Acanthodians Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...

, the class ''Ischnacanthus'' is home to, are the subject of some dispute over their systematic position because they have features of both bony fish, the

Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...

, and cartilaginous fish, the

Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissu ...

Description

''Ischnacanthus'' was a fish in the class

that had a long body covered in mosaic-like scales. They possess highly advanced, spindle-shaped bodies that were thought to have made them swift swimmers. This fish had two narrow dorsal spines, one either side of and just behind its head. It was a predatory fish that possessed a mouth with very small teeth on the lower jaw. The feature all

Acanthodian Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...

s share in common is the fact that massive spines, formed of dentine, support all fins other than the caudal fins. This species probably lived in

Lake Forfar A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

, which is a fresh-water lake surrounded by volcanoes.

References

Acanthodii genera Early Devonian fish Devonian acanthodians Extinct animals of Europe {{Devonian-fish-stub