#jaw evolution

• Acanthodian — A fossil representative of the extinct class Acanthodii, a group of ancient jawed vertebrates (gnathostomes), considered among the earliest to possess jaws, which inhabited global waters from the Silurian to the Permian periods. Popularly termed 'spiny sharks', these small primitive fish were characterized by a partially cartilaginous skeleton and, definitively, by the presence of stout, rigid bony spines located at the leading edge of all their fins, serving for structural support and predator defense.
• Acanthodii — An extinct class of early bony fishes (Acanthodii) from the Paleozoic era, recognized as the earliest jawed vertebrates in the fossil record. The organisms were individualized by their generally small size, a streamlined body protected by tough rhombic scales, and the characteristic presence of long, rigid bony spines located at the base of both paired and unpaired fins. Due to their intermediate traits, which blend anatomical features of sharks and modern bony fishes, they are commonly termed 'spiny sharks' in literature.