Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A): All vertebrates are chordates but all chordates are not vertebrate.

Reason (R): The members of subphylum vertebrata possess notochord, during the embryonic period, the notochord is replaced by a cartilaginous or bony vertebral column in adults.

In the light of the above statements, choose the correct answer from the options given below:

The correct option is Both A and R are true and R is the correct explanation of A.

Analysis of Assertion (A):
Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata. All members of these subphyla are chordates because they possess a notochord at some stage of their lifecycle. However, only the members of the subphylum Vertebrata develop a vertebral column. The chordates belonging to Urochordata and Cephalochordata do not develop a vertebral column. Therefore, all vertebrates are chordates, but not all chordates are vertebrates. Thus, Assertion (A) is true.

Analysis of Reason (R):
In vertebrates, the notochord is present during embryonic development, but as the organism matures, it is replaced by a cartilaginous or bony vertebral column in adults. Since this replacement of the notochord with a vertebral column is the defining characteristic that distinguishes vertebrates from other chordates (protochordates), it explains why only a subset of chordates become vertebrates. Thus, Reason (R) is true and is the correct explanation of Assertion (A).