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.

The notochord is a flexible rod found in the embryos of all chordates, and is replaced later in life by the vertebral column and the main axial support of the body. If the animal you are referring to is an adult and lacking a "true notochord", it is still a chordate because the notochord has simply transformed into other parts of the body. However, if it is still an embryo and STILL has no notochord, it is not a true chordate.