Cartilaginous Fish

What are they?

Sharks, rays, and other stranger fish make up the Chondrichthyes, or "cartilaginous fish." Their first appearance on the face of this Earth occured almost 450 million years ago. Cartilaginous fish include both predators and mollusc-eaters.

Characteristics

Members of this particular branch under fish all lack what can be called true bone and have a skeleton made of cartilage which is the flexible material found in the nose and ears. Only their teeth, and sometimes their vertebrae, are calcified and can be found as fossils; this calcified cartilage has a different structure from that of true bone. Thus, preservation of the whole body of a cartilaginous fish only takes place under special conditions especially as they are not the normal sort of solid bones.

Most sharks feed on fish, squid, and marine mammals - although comparatively few are dangerous to humans and do not attack humans often. However, other cartilaginous fishes, namely skates. rays and chimaeras, live on crustaceans and molluscs the reason being that they lack the sharp daggerlike teeth of predatory sharks, and their teeth are in the form of heavy crushing plates. But not all sharks have a feature of being predators - several large sharks, have small teeth and hence their main source of food is plankton. As they swim in the open ocean, water is taken in through the mouth and strained through the gill slits. The largest shark of all, the whale shark, comes under such plankton feeders.

Traditional classification divides this group ( Chondrichthyes ) into two sub groups. The

Elasmobranchii includes the familiar fishes like the sharks, skates, and rays, as well as some strange fossil relatives. The features that are found in this group is that they an upper jaw not fused to the braincase and separate slitlike gill openings.

The second sub group, the Bradyodonti, includes features like an upper jaw fused to the braincase and a flap of skin, ( called the operculum ), covering the gill slits - protecting. This group includes fishes like chimaeras and ratfish, which are relatively hard-to-find, deep-water, mollusc-eating fishes.

From where did they evolve and when?

Not Precisely known.