Sharks, in their basal form have been on Earth for hundreds of millions of years. They were the first to develop a stream-like, torpedo shaped body and created a niche for themselves. Their aerodynamic body shape has evolved to allow for efficient and dynamic movement in water. Having a cartilage instead of bones allowed them to grow large and still be relatively light and buoyant . After they've evolved flexible, protruding jaws, there wasn't much to change. They were able to adapt and exploit all the water habitats as the world was changing.
