The ancient relatives of modern sharks appeared on earth about 400 million years ago in the Devonian period. At this time the earliest insects flew around in primitive plant forests and the oceans were filled with invertebrate animals, such as mollusks, sea lilies and other, now extinct animal groups. These favorable conditions made it possible for new types of animals to evolve and inhabit the worlds oceans and fresh waters. One of these new animal groups were the fishes and the ancestors of today's sharks.

One of the earliest sharks to appear were the members of the genus Cladoselache, which shared many of the features found in modern sharks. They had five gill slits, two well developed dorsal fins and a skeleton made out of cartilage. The Cladoselache could be up to 2 meters long and were probably a good swimmer feeding primarily on small fishes. Unlike today's sharks the Cladoselache lacked anal fin and external reproductive organs. Although these sharks were abundant in the Devonian seas they are probably not direct ancestors of the modern sharks, but instead un offshoot of the evolutionary line that gave rise to today's sharks.


(© J. Lokrantz/Azoteimages)

By the end of the Carboniferous, 290 million years ago, the Cladoselache sharks disappeared and new, more advanced sharks took their place. One of the most important groups were the hybodonts. These sharks more closely resembled modern sharks in general appearance. They had an anal fin and external reproductive organs (claspers) and they also had two different types of teeth. Sharp teeth in the front of the jaw for catching fishes and flat crushing ones in the rear for shelled invertebrates. The hybodonts were generalists witch made it possible for them to occupy a wide range of niches. According to their bodily construction they were no typical fast swimmer nor specialized bottom dwellers. The hybodonts flourished until the end of Cretaceous, 65 million years ago, when they became extinct together with the dinosaurs.


(© J. Lokrantz/Azoteimages)

During the prime time of the hybodonts, approx. 150 million years ago, most modern sharks made their appearance. Among the earliest sharks to appear in the fossil records are the sixgill and sevengill sharks (order Hexanchiformes, cowsharks) witch are easily distinguished by the presence of a single dorsal fin instead of two. During this time most of the other modern shark families also evolved.

One of the most interesting prehistoric species to appear were the Carcharodon megalodon. This giant shark appears in the fossil records 25 million years ago, and roamed the seas until 3 million years ago. The C. megalodon are the largest shark ever to have lived on earth and are probably closely related to the living great white shark, Carcharodon carcharias. The teeth from C. megalodon can be up to 15 cm high and reconstruction of the jaw indicate that this species could reach total length of approximately 15 meters and weigh more than 25 tons.