<Tr> <Th> Cladistically included but traditionally excluded groups </Th> </Tr> <Tr> <Td> <P> Tetrapods </P> </Td> </Tr> <P> The evolution of fish began about 530 million years ago during the Cambrian explosion . It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates . The first fish lineages belong to the Agnatha, or jawless fish . Early examples include Haikouichthys . During the late Cambrian, eel - like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared . Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish . Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans . </P> <P> The first jawed vertebrates probably developed during the late Ordovician period . They are first represented in the fossil record from the Silurian by two groups of fish: the armoured fish known as placoderms, which evolved from the ostracoderms; and the Acanthodii (or spiny sharks). The jawed fish that are still extant in modern days also appeared in late Silurian: the Chondrichthyes (or cartilaginous fish) and the Osteichthyes (or bony fish). The bony fish evolved into two separate groups: the Actinopterygii (or ray - finned fish) and Sarcopterygii (which includes the lobe - finned fish). </P>

Fish appeared in the fossil record during the