Until about 400 million years ago, fishes -- the first true swimmers -- were jawless  Their feeding methods were limited to either sucking in surface plankton or sucking in food particles from bottom mud  With the development of biting jaws, however, the life of fishes changed dramatically, since jaws allowed them actively to pursue prey, to seize it in their jaws, and to manipulate it between their teeth  The jawed fishes then developed along two main lines: one retained cartilage for its skeletons, for example, sharks and rays; the other adopted bone as its principal skeletal material
From the latter group evolved the most abundant and diverse of all of today' s vertebrate groups, the teleosts, some 21, 000 species, which vary from barracudas to sea horses
Jawless fishes did not have cartilage as their skeletal material.