A shark's cartilaginous skeleton

Shark skeletons are made mostly of cartilage rather than true bone; cartilage is lighter and more flexible, though some regions can be reinforced with minerals. A shark’s teeth are not cartilage but hard, tooth-like tissue. Many species continually replace their teeth. Sharks have existed for hundreds of millions of years, but their evolutionary success cannot be attributed to one set of traits alone; diversity and adaptation to different environments also matter.