Bat rays evolved along with sharks. This group of animals is particularly special, because they have an impressive internal structure not of calcified bone, but of cartilage. Cartilage gives these animals several advantages. They are less dense, so it’s easier to stay afloat. This helps to avoid the need for the swim bladders which most bony fish (teleosts) have. It also makes them more flexible, which is good when needing to make tight turns, while hunting prey. In the areas where greater skeletal strength is needed such as powerful jaws, calcium salt is used, but it’s still lighter than a calcium-rich bone.