Acanthodii

Acanthodii, commonly known as "spiny sharks," are an extinct class of early jawed fishes that lived from the Silurian to the Permian period. Despite their name, they are not true sharks but share some features, like a streamlined body and multiple pairs of fins. They usually had rows of sharp spines along their back and fins, which likely helped with stability and defense. Acanthodii are important for understanding the evolution of modern fish, as they bridge the gap between primitive jawless fishes and more advanced jawed fishes.