Fish first appear in the early Cambrian (~525 Ma) of China. These are chordates characterized by a notochord (a stiffened rod) a dorsal nerve cord, chevron folded muscles. The early chordates lacked jaws (they were “agnathans” “no jaws”) and bone and resemble modern “Amphioxis” [now called Branchiostoma].
[Interlude: Amphioxis song]
The conodonts appear in the early Cambrian ~525 Ma still lack bone but have
mineralized tooth-like elements arranged in 7 batteries of specialized structures.
Wear patterns suggest they were used as teeth rather than supporting a feeding structure and
they range from stabbing forms at the front of the mouth to crushing forms at the rear, further demonstrating their use as teeth;
However conodont animals had no jaws. Large eyes suggest these were visual predators. Conodonts were likely pelagic predators since they:
are very widespread (and used for biostratigraphy)
small (a few centimeters, max),
are found commonly in relatively deep ocean sediments