Stethacanthus altonensis, like the other stethacanthid sharks, displayed conspicuous sexual dimorphism. Males had large spike-like denticles on top of head and the crest of the highly modified first dorsal fin. The fin and spine of S. altonensis are back-to-back upright triangles with the dorsal fin movably hinged to the rear margin of the spine by a tongue-in-groove joint. (The illustration of the spine is a redrafting of the original drawing of the type specimen). The spine itself was not movable, being mounted deep between the epaxial muscles. Females lacked the fin and the spine, as well as any scales at all.