A subphylum of the phylum Chordata characterized by possession of a segmented backbone or spinal column. It includes the following classes: Agnatha (cyclostomes), Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia, Reptilia, Aves, and Mammalia. Members of this subphylum possess an axial notochord at some period of their existence.