Haikouichthys

Haikouichthys was one of the first organisms to have a distinct backbone and head, making it a notable transitional form in the transition from invertebrates to fish. It arose during the Cambrian explosion around 530 million years ago, along with  and various Though it was likely not a direct ancestor of fishes, it is often referred to as one. Its backbone was a notochord, that is, a cartilaginous rod running down its back, and it had gill slits, as with all other primitive chordates.