Ans.
The first jawless fishes were the first fishes of earth, evolved during the Ordovician period. These fishes belong to phylum Chordata and subphylum Vertebrata, and are known as agnathans as they lack jaws. Other characteristics of jawless fishes include presence of cartilaginous skeleton, paired gill pouches, and notochord.
Thus, the correct answer is option A). 'vertebrates.'