One of the larger orders of fossil fish belonging to the class Acanthodii. They possessed bony jaws and body skeleton, ganoid scales, a heterocercal tail, and stout spines located before the dorsal, anal, pectoral, and pelvic fins. One of the better-known forms, Climatius, carried five additional pairs of fins along the belly and reached a length of only 8 cm. These ‘spiny sharks’ may actually be half-way between the sharks and the bony fish (Osteichthyes).