Elasmobranchii

Elasmobranchii is a subclass of cartilaginous fish, which includes modern sharks, rays, skates, and sawfish. These fish are special because they have five to seven pairs of gill slits that open to the outside, rigid dorsal fins, and tiny placoid scales on their skin. Their teeth are arranged in many series, and their upper jaw is not fused to the skull, which helps scientists tell different species apart.

Male elasmobranchs have pelvic fins changed into claspers, which help them transfer sperm during reproduction. Unlike some other fish, elasmobranchs do not have swim bladders to help them stay afloat. Instead, they have large livers filled with oil, which gives them buoyancy.

The name Elasmobranchii comes from Ancient Greek words meaning "plate" and "gill," referring to their broad, flat gills. Some deep-sea sharks, like the school, gulper, and basking sharks, are caught for their valuable liver oil. Sadly, these species are vulnerable because of overfishing, and like other animals with similar life patterns, they are easily affected by fishing pressure.

Description

Elasmobranchs, such as sharks, rays, and skates, have special features that set them apart from other fish. They have five to seven pairs of gills that open directly to the outside, rigid dorsal fins, and tiny, hard scales called placoid scales on their skin. Their teeth come in many rows, and their upper jaw is not connected to the skull, while the lower jaw moves to help them chew.

These fish do not have swim bladders to help them float. Instead, they stay buoyant with large livers filled with oil, which can also provide energy when food is hard to find. Males have special structures called claspers near their pelvic fins to help with reproduction. Elasmobranchs live in both tropical and temperate waters around the world.

Evolutionary history

See also: Evolution of fish

The oldest known member of the elasmobranch group, Phoebodus, appeared in the Middle Devonian period, about 383 million years ago. By the end of the Devonian, important groups like Ctenacanthiformes and Hybodontiformes had already evolved. During the Carboniferous period, some ctenacanths grew as large as modern great white sharks, reaching lengths of about 7 meters (23 feet). In both the Carboniferous and Permian periods, xenacanths were common in rivers and oceans and continued into the Triassic period with fewer types. Hybodonts became very diverse by the Permian and were the main group of elasmobranchs during the Triassic and Early Jurassic periods. They lived in both saltwater and freshwater.

Modern sharks and rays began to appear in the Triassic but didn’t become very diverse until the Early Jurassic. This rise of modern sharks and rays happened as hybodonts became less common in oceans by the Late Jurassic, though they stayed in rivers into the Cretaceous period. The last known hybodonts lived at the very end of the Cretaceous.

Taxonomy

Elasmobranchii is a group of fish that was first named in 1838. It includes modern sharks and rays, as well as many ancient fish that are now extinct. Scientists have debated how to group these fish, but they agree that sharks and rays share a common ancestor.

The group Elasmobranchii includes many different kinds of sharks and rays. Some scientists think that rays and skates are closely related to sharks, while others believe they form their own group within Elasmobranchii.