Interesting. Sharks may have evolved to become faster swimmers by losing bone, replacing it with cartilage.
"Cartilage doesn’t preserve as well as bones, so the early shark fossil records are based mostly on isolated scales and teeth.
This poor fossil record is partly responsible for scientists thinking that sharks must represent a primitive condition in vertebrate evolution compared to all other fishes and land animals (tetrapods) which have a well-ossified bony skeleton.
But this idea has just been challenged due to the discovery, announced today in the journal PLOS One [ http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0126066
], of a 380-million-year-old fossil shark from Western Australia named Gogoselachus lynbeazleyae
that shows remnant bone cells present in its cartilaginous skeleton.
This implied that sharks most likely evolved from ancestors that had much more bone in the skeleton. The evolution of modern sharks was driven by their loss of bone, which suggested they are not as primitive as previously thought."