Sharks are a group of elasmobranch 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 Batoidea (rays and kin). Some sources extend the term “shark“ as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts and xenacanths. Shark-like chondrichthyans such as Cladoselache and Doliodus first appeared in the Devonian Period (419-359 Ma), though some fossilized chondrichthyan-like scales are as old as the Late Ordovician (458-444 Ma). The oldest modern sharks (selachians) are known from the Early Jurassic, about 200 Ma.