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 Anomalocaris
and various trilobites
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.