The jawless fish are the oldest of any vertebrate on earth and can be found as fossil fish dating back more than several hundred million years old. The jawless fish, also known as Agnatha lack real jaws or paired fins. Some of these fish species have survived today with a good example being the lamprey.

The sharks, skates and rays in the class Chondricthyes are fish with no bones and only cartilage. Sharks, skates and rays have gills slits that are open and many of them have large teeth for predation. Most often, the teeth of these fish are found as fossils. Fossil teeth of sharks are extremely common in the fossil record and are an indication of their long term success in the oceans.

Placoderms or armored fish were not as lucky as the two classes mentioned above and became extinct during the fossil record. These fossils can be found with simple jaws, paired fins, bony bodies, armor headshields and thick spiny scales. Although extinct now, the Placoderm fossil fish date many hundreds of millions of years back in the fossil record.

The most well-known fishes today are the bony fishes in the class Osteichthyes. These fish have been so successful throughout evolution that they have become the most complicated and widely spread class of fishes. All bony fishes have bones instead of cartilage to support their body. They also have scales, paired fins and a highly adapted tail.