Fishes, which are simply defined as all vertebrates except those with legs, are members of the chordate phylum.
As such, they display certain characteristic features: a skeletal rod called a notochord, a dorsal nerve, gills, and a tail.
Sharks today are "living fossils"; they have changed somewhat in appearance over the years, but their predatory ways have not.
Over two pulses spaced a million years apart, highly diverse marine groups die out.
Sharks, with their cousins the skates and rays, belong to a group of fishes whose skeletons are made of cartilage, not bone.