The assertion is true. Vertebrates are a subphylum within the phylum Chordata. However, other chordate subphyla (Urochordata and Cephalochordata) are not vertebrates. The reason is also true – vertebrates develop a vertebral column that replaces the notochord. However, the reason doesn't fully explain why all chordates aren't vertebrates. The key difference lies in the development of a vertebral column, which is unique to vertebrates. Other chordates retain the notochord throughout their lives.