Shark is the naming term of all members of Selachimorpha suborder in the subclass Elasmobranchii, in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. The first shark-like chondrichthyans appeared in the oceans 400 million years ago, developing into the crown group of sharks by the Early Jurassic.