information:highlighting
Vertebral column

With only one exception, the defining characteristic of all vertebrate is the vertebral column, in which the embryonic notochord found in all chordates is replaced by a segmented series of mineralized elements called vertebrae separated by fibrocartilaginous intervertebral discs, which are derived embryonically and evolutionarily from the notochord. Hagfish are the only extant vertebrate whose notochord persists and is not integrated/ replaced by the vertebral column.

A few vertebrates have secondarily lost this feature and retain the notochord into adulthood, such as the sturgeon and coelacanth. Jawed vertebrates are typified by paired appendages (fins or limbs, which may be secondarily lost), but this trait is not required to qualify an animal as a vertebrate.