Ischyodus
Ischyodus (from , 'power' and 'tooth') is an extinct genus of cartilaginous fish belonging to the subclass Holocephali, which includes the modern-day chimaeras. Fossils are known from Europe (including Russia), North America, and New Zealand. "Ischyodus" was rather similar 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 dorsal spike attached to the front of the dorsal fin. The spike probably served as a method of protection against  predators, and may have been venomous, as it is in modern chimaeras. Dental plates of at least two species, "Ischyodus rayhaasi" and "Ischyodus dolloi", have been found at several sites in North Dakota. Based on complete specimen, total length is 21 times larger than mesiodistal length of mandibular tooth plate. For example, "I. bifurcatus" with mandibular plate length of possibly belongs long specimen.