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. 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.