Notochord.

You did. Your backbone formed on the notochord. There are some fish, the lamprey, I think, that retain a notochord, These animals are chordates but not vertebrates.

The notochord is a characteristic of the 9th & 10th division of the kingdom animalia ,i.e., Protochordata and Vertebra

A chordoma is a rare type of cancer that develops from the notochord. The notochord forms the early spine in a foetus as it develops inside the womb. After about six months, most of the notochord is replaced by the bones of the spine. However, small amounts of the notochord may remain, and these can sometimes develop into a chordoma.

No, an animal with a backbone and gills but no notochord would not be considered a chordate. Chordates are defined by the presence of a notochord at some stage of their development, along with a dorsal nerve cord, pharyngeal slits, and a post-anal tail.