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 answer is Both A and R are true, and R is the correct explanation of A

Concept:

• Chordates are animals that possess, at some stage of their life cycle, a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail.
• The notochord is a flexible rod that provides support in all embryonic and some adult chordate animals.
• In vertebrates, the notochord is present during embryonic development but is later replaced by the vertebral column (spine) as the main structural support.

Explanation:

• Assertion (A): The statement "All vertebrates are chordates but all chordates are not vertebrates" is true. This is because vertebrates form a subgroup within the larger group of chordates.
• Reason (R): The statement "The members of subphylum Vertebrata possess a notochord, during the embryonic period, the notochord is replaced by a cartilaginous or bony vertebral column in adults" is also true. Vertebrates initially develop a notochord in their embryonic stage which is later replaced by a vertebral column made of cartilage or bone.

 The Reason (R) correctly explains the Assertion (A) because it details a key characteristic that distinguishes vertebrates from other chordates – the replacement of the notochord by a vertebral column.