Sharks are a group of
fish characterized by a
cartilaginous skeleton, five to seven
gill slits on the sides of the head, and
pectoral 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 subclass
Elasmobranchii outside the Selachimorpha, such as
Cladoselache and
Xenacanthus, as well as other
Chondrichthyes such as the
holocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago.
Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole.
