Ischnacanthus is an extinct genus of fish in the class Acanthodii. Ischnacanthus lived during the Early Devonian period. Ischnacanthus has jaws but still retains small mosaic scales. They either had very small teeth or none. Fossils were found in Tillywhandland Quarry, Forfar, Scotland. Some species were of large size, up to 2 m in length.