[2] Holoptychius was a streamlined predator about 50 centimetres (20 in) long (though largest specimen could grew up to 2.5 metres (8.2 ft)), which fed on other bony fish.

Its rounded scales and body form indicate that it could have swum quickly through the water to catch prey.

Its asymmetrical tail sported a caudal fin on its lower end.

Of the genus Holoptychius the following species have been described:[2] Fossils of Holoptychius have been found in the Mississippian of the United Kingdom and the Devonian of Belgium, Colombia (Cuche Formation, Boyacá),[8][7] Norway, Canada,[5][6] the Russian Federation, and the United States (Pennsylvania).

[9] Isotopic analysis of Holoptychius remains from Greenland reveals that it was likely a euryhaline organism.