Elasmobranchii

Elasmobranchii is a group of special fish. It includes modern sharks, rays, skates, and sawfish. These fish have five to seven pairs of gill slits that open to the outside. They also have 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.

Male elasmobranchs have special pelvic fins called claspers. These help them during reproduction. Unlike some other fish, elasmobranchs do not have swim bladders. Instead, they have large livers filled with oil, which helps them stay afloat.

The name Elasmobranchii comes from Ancient Greek words meaning "plate" and "gill." This refers to their broad, flat gills. Some deep-sea sharks are caught for their valuable liver oil. Sadly, these species are vulnerable because of overfishing.

Description

Elasmobranchs, such as sharks, rays, and skates, have special features that make them different from other fish. They have five to seven pairs of gills that open directly to the outside, rigid dorsal fins, and tiny, hard 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 evolved. During the Carboniferous period, some ctenacanths grew very large, about as long as modern great white sharks. In the Carboniferous and Permian periods, xenacanths lived in rivers and oceans. They 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 named in 1838. It includes modern sharks and rays, as well as many ancient fish that are no longer alive. Scientists discuss how to group these fish, but they agree sharks and rays had a common ancestor.

The group Elasmobranchii has many kinds of sharks and rays. Some scientists think rays and skates are close to sharks. Others think they are their own group inside Elasmobranchii.