Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, andpectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii), and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the suborderElasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago. Since that time, sharks have diversified into over 400 species.
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families. They are closely related to sharks, from which they can be distinguished by their flattened bodies, enlarged pectoral fins that are fused to the head, and gill slits that are placed on their ventral surfaces.
Skates are cartilaginous fishes belonging to the family Rajidae in the superorder Batoidea of rays. More than 200 species have been described, in 27 genera. There are two subfamilies, Rajinae (hardnose skates) and Arhynchobatinae (softnose skates).
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish (not to be confused with the rattails), spookfish (not to be confused with the true spookfish of the family Opisthoproctidae), or rabbitfishes (not to be confused with the true rabbitfishes of the family Siganidae). They may be the "oldest and most enigmatic groups of fishes alive today." At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though in evolutionary terms they branched off from sharks nearly 400 million years ago and have remained isolated ever since. Today they are largely confined to deep water.