Euselachii

Euselachii is a subclass of cartilaginous fish that includes all modern sharks, rays, and skates. The name Euselachii is derived from the Greek words "eu" meaning "true" and "selachos" meaning "shark." This subclass is characterized by a skeleton made of cartilage instead of bone, placoid scales covering their skin, and a hinged jaw that allows them to open their mouths wide.

Fossils attributed to Euselachii date back to the Devonian period, over 400 million years ago. Over time, this group diversified into a wide array of species adapted to various marine environments, from shallow coastal waters to the deep ocean. Modern euselachians are typically predators, with diets varying based on species, ranging from plankton to larger fish and marine mammals.

The reproductive strategies within Euselachii are diverse, including oviparous (egg-laying), viviparous (live-bearing with a placenta), and ovoviviparous (eggs hatch inside the mother). They possess a keen sense of smell and can detect electrical fields generated by prey using specialized organs called ampullae of Lorenzini. Despite their ancient lineage and ecological importance, many euselachian species are currently facing threats from overfishing and habitat degradation, leading to declining populations worldwide.