Ischyodus is an extinct genus of cartilaginous fish belonging to the subclass Holocephali, which includes the modern-day chimaeras. Ischyodus was virtually identical to the present-day chimaera Chimaera monstrosa, which is found in the Atlantic Ocean and Mediterranean Sea. Just like C. monstrosa, Ischyodus had large eyes, a long whip-like tail, small lips, large pectoral fins and dorsal fin, and a poisonous dorsal spike attached to the front of the dorsal fin. The spike probably served as a method of protection against predators. Dental plates have been found at several sites in North Dakota.