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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.[1] It arose during the Cambrian explosion around 530 million years ago, along with AnomalocarisWikipedia's W.svg and various trilobites.Wikipedia's W.svg Though it was likely not a direct ancestor of fishes, it is often referred to as one.[2][3] 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.