Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the division
Selachii and are the sister group to the Batoidea. Some sources extend the term "shark" as an informal category including extinct members of Chondrichthyes with a shark-like morphology, such as hybodonts. Shark-like chondrichthyans such as
Cladoselache and
Doliodus first appeared in the Devonian Period, though some fossilized chondrichthyan-like scales are as old as the Late Ordovician. The earliest confirmed modern sharks (selachimorphs) are known from the Early Jurassic around
200 million years ago, with the oldest known member being
Agaleus, though records of true sharks may extend back as far as the Permian.