These animals hunted along the seabed ... Fossils found in the Burgess Shale of British Columbia show traces of a notochord (a rodlike primitive backbone), a significant step in the evolution ...

Many four-legged animals go through embryonic stages that have similar features -- gill arches, a notochord, segmentation, and paddle-like limb buds -- as they develop into different adults.

including the presence of a notochord, which is a chord-like central nerve bundle like a spinal column, but without bones.