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 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 answer is: Both (A) and (R) are true and (R) is the correct explanation of (A).

Explanation of the Assertion (A)
All vertebrates belong to the subphylum Vertebrata, which is a subgroup of the phylum Chordata. Therefore every vertebrate is a chordate. However, the chordate phylum also contains other subphyla such as Cephalochordata (lancelets) and Urochordata (tunicates) that are not vertebrates. Hence, “all vertebrates are chordates but not all chordates are vertebrates” is a true statement.

Explanation of the Reason (R)
Members of the subphylum Vertebrata possess a notochord during the embryonic stage. During later development the notochord is replaced by a cartilaginous or bony vertebral column (the backbone) in the adult animal. This developmental fact is accurate and describes a key characteristic of vertebrates.

Why (R) correctly explains (A)
The reason given describes a distinctive feature of vertebrates (the temporary presence of a notochord). Because vertebrates develop from chordate ancestors that initially have a notochord, they are classified as chordates. The fact that only vertebrates later replace the notochord with a vertebral column distinguishes them from other chordates, which retain the notochord throughout life. Thus the information in (R) provides the biological basis for the relationship stated in (A). Consequently, both statements are true and (R) serves as the correct explanation for (A).