Shark skeletons are composed of cartilage. Cartilage, the stuff your ears and nose are made of, is much softer than bone, and rarely survives fossilization. Because of this, entire fossilized skeletons of sharks are, generally, a rare find. The vertebral centra (centers of vertebra) are the densest part of the shark skeleton and sometimes fossilize. Vertebral centra are disk shaped. Measures 37x36mm.