To which biological phylum do all mammals, birds and reptiles belong?

All mammals, birds, and reptiles belong to the phylum Chordata because they possess a unique structure called a notochord during some stage of their development. This notochord is a flexible, rod-like structure that provides support for the body and eventually develops into a backbone or vertebral column in most members of this phylum. Additionally, Chordata organisms also possess a nerve cord that runs along their back and a series of gill slits or pouches at some point in their lives. These features, along with other characteristics such as having a bilateral symmetry and a dorsal hollow nerve cord, classify mammals, birds, and reptiles into the phylum Chordata.