Re-inventing the cartilage skeleton
At this time a pronounced split appeared in the fish dynasty, with one line of animals losing all their bone and developing cartilage, a softer more elastic and lighter material. The descendants of this are the fish belonging to the class Chondrichthyes and represented by sharks (orders Galeomorpha and Squalomorpha), rays (order Batoidea) and chimeras (order Chimaerida). Although this lightened them they would still need to continue to swim or they would sink. Swimming is still accompanied by a powerful thrash of the tail and pectoral fins which prevent them from diving nose down. Since the pectoral fin is stiff these have less mobility than the pectoral fins of the bony fish. Some of these fish rested by sinking to the sea-floor, and one group has adopted such a position on a semi-permanent basis (rays and skates). As a consequence they have become greatly flattened with pectoral fins expanded into undulating lateral triangles which they use for locomotion and the muscle in the tail is almost completely lost (although it may bear a poisonous spine at the end). Rays and skates are not as fast swimming as sharks, but this is of less importance since they feed on molluscs and crustaceans.
The manta ray, or giant manta (Manta birostris), is the largest of the rays, ranging up to 6.7 meters (22 ft) across its pectoral fins (or "wings") and weighing up to 1,350 kg (3,000 lb). It ranges throughout the tropical seas of the world, typically around coral reefs. Mantas are most commonly black above and white below, but some are blue on their backs. A giant manta's eyes are located at the base of the cephalic fins on each side of the head, and unlike other rays the mouth is found at the anterior edge of its head. To breathe, the manta has like other rays five pair of gills on the underside