In life, it would have resembled a compressed boxfish with five massive spines along the anterior-dorsal side, with the longest spine directly above the forehead, and the shortest spine directly in front of the dorsal fin. It is distinguished from its close, sympatric relative, Spinacanthus, in that its scales are large, and form a sort of armor. (InS. cuneiformis, the individual scales are relatively small, and do not touch each other).
Protobalistum imperiale and Spinacanthus were a part of the ecosystem of the lagoon that would become Monte Bolca. It has been suggested that, because of their similarity to boxfish, and due to their close relation to modern-day triggerfish, spinacanthids may have preyed on shellfish and small fish.
