Neoselachians

Neoselachians are a infraclass of cartilaginous fish that comprises all modern sharks, rays, and skates. This group is distinct from the extinct hybodontiform sharks and the early sharks of the Paleozoic era. Neoselachians are characterized by a number of skeletal and soft-tissue features, including a more complex jaw structure and the presence of five to seven gill slits. Their evolutionary history traces back to the Triassic period, and they have undergone significant diversification to occupy a wide range of marine and some freshwater environments.

The key distinguishing feature of neoselachians is their jaw suspension, which allows for greater protrusibility of the jaws, a critical adaptation for predation. Internally, they possess a spiral valve in their intestine, which increases surface area for nutrient absorption. Externally, their skin is covered in placoid scales, often called dermal denticles, which are structurally similar to teeth and contribute to their streamlined form. This infraclass is broadly divided into two superorders: Selachimorpha (sharks) and Batoidea (rays and skates). While sharks are typically more fusiform and possess distinct dorsal fins, rays and skates are generally flattened dorsoventrally, with greatly enlarged pectoral fins fused to the head, adapted for life on the seabed. Despite these morphological differences, they share a common ancestry within the neoselachian lineage.