Previously, known specimens of the Lochkovian (Lower Devonian) diplacanthid acanthodian genus Tetanopsyrus (all from the MOTH locality in the Northwest Territories, Canada) were thought to belong to a single species, T. lindoei. New specimens from the same locality have shown that two species are present. Tetanopsyrus lindoei is revised and T. breviacanthias, sp. nov. is described, leading to a revised diagnosis of the genus. Both species lack rostral tesserae; instead they have irregularly-shaped, monodontode, rostral scales without basal tissue and with an open pulp cavity. Both species also have enlarged, tuberculated, anterior circumorbital plates. Dorsal spines are shallowly inserted and each is supported by a basal plate that ossified early in ontogeny. The two species are distinguished from each other by the structure of the dorsal spines, the length of the pectoral spines, and the shape of the procoracoids.
The revision of Tetanopsyrus has implications also for relationships among acanthodian families. Both Tetanopsyrus (Tetanopsyridae) and Gladiobranchus (Gladiobranchidae) are seen as diplacanthiforms rather than as ischnacanthiforms. Similarities are found in the jaws, circumorbital plates, anterior dorsal spine, dorsal spine basal plates, prepelvic spines, and scapulocoracoids. Other potential relatives of diplacanthids include Uraniacanthus, which has similarities in prepelvic and dorsal spines, and Culmacanthus, whose relationships cannot be settled without further study.