Jaws probably originated in the pharyngeal arches supporting the gills of jawless fish. Lampreys form a superclass containing 38 known extant species of jawless fish. It is present in all vertebrates above the jawless fish. Pteraspidomorphi is an extinct class of early jawless fish. Zenaspida is an extinct order of jawless fish. The first fish lineages belong to the Agnatha, or jawless fish. Pteraspidomorphi is an extinct class of early jawless fish ancestral to jawed vertebrates. Cephalaspidida is an extinct order of jawless fish in the subclass Cornuata. Extant jawless fish are either lamprey or hagfish. Jawless fish were the earliest fish to evolve. Sacabambaspis is an extinct genus of jawless fish that lived in the Ordovician period. Contemporary vertebrates included armored jawless fish called ostracoderms. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms. Some fish, both bony fish (Osteichthyes) and jawless fish (Agnatha), undergo metamorphosis. In the jawless fish (lampreys and hagfish), true lymphoid organs are absent.