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.
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