Fossil record

Sharks, with their cousins the skates and rays, belong to a group of fishes whose skeletons are made of cartilage, not bone. While sharks are not plentiful until the Devonian period and later, fossil scales date the earliest sharks to the late Ordovician. Sharks today are "living fossils"; they have changed somewhat in appearance over the years, but their predatory ways have not.

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. Agnathans, or jawless fishes, are the earliest fishes and the first true vertebrates. They are bottom-feeders, covered almost entirely in armor plates. As jaws evolve in the bony fishes and early sharks, jawless fishes have trouble competing. Hagfishes and lampreys are the only jawless fishes alive today.