Sharks do not have a backbone because their skeletal system is made up of a softer cartilage, just like all the other fish that belong to the Chondrichthyes class. The cartilage found in sharks is not as hard as a real bone, but it is strong enough to support their muscular system and the rest of their body.
Because cartilage is softer than real bones, shark fossils are seldom found completely intact. There are around 400 different species of sharks in the world, with new species being found every day. Not much data is available for exactly how long all species of sharks live, but one shark, which is the whale shark, can live for over 100 years.