Selachimorpha
Sharks are a group of elasmobranch fish characterized by their unique features, including a cartilaginous skeleton, five to seven gill slits on the sides of the head, and separate pectoral fins. These fascinating creatures belong to the clade Selachimorpha and are closely related to the Batoidea. It's worth mentioning that in some cases, the term "shark" is informally used to include extinct members of Chondrichthyes that share shark-like characteristics, such as hybodonts.
Chondrichthyans resembling sharks, such as Cladoselache and Doliodus, first appeared during the Devonian Period. However, the discovery of fossilized chondrichthyan-like scales dating back to the Late Ordovician suggests an even older existence. The earliest confirmed modern sharks (selachimorphs) are known from the Early Jurassic period, approximately 200 million years ago. However, it is possible that true sharks have a lineage that predates this period and can be traced back to the Permian era.
