^w^ A shark is distinguished from other fish by its cartilaginous skeleton, its scales, and its mode of reproduction.
^w^ The chondrichthyes, or cartilaginous fish, comprise two groups: 380 diverse species of sharks, 420 species of rays and skates (together called Elasmobranchs) and a smaller group of cartilaginous fish called chimaeras.
^w^ Sharks can be traced back over 400 million years in the fossil record, but modern sharks share fin, jaw and tooth forms with ancestors that can be traced back only about 180 million years.
^w^ Sharks may have reached their peak size and numbers during the Miocene period, 15 to 25 million years ago. During the height of their diversity, sharks and their relatives made up over 60% of the species of fish.
