A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth. Most acanthodians have strong spines at the leading edges of all fins except for the caudal (tail) fin. Acanthodians mostly were small fishes with large mouths and large eyes, suggesting an active, predatory lifestyle feeding on small prey, including other fishes. The oldest well-preserved examples are of Early Silurian age, and the last surviving examples are of Early Permian age. Primitive forms tended to be from marine environments and to have stouter bodies, larger stouter fin spines, and larger numbers of paired spines in front of the pelvic fins. Later forms are found also in freshwater and estuarine habitats and had more slender bodies with fewer and more slender spines.