Chordate Zoology - Textbook - Y. V. Tsaryk - 2013

Chapter 4. DIVISION GNATHOSTOMES, or ECTOBRANCHIATES, GNATHOSTOMATA, seu ECTОBRANCHIATA. SUPERCLASS FISHES, PISCES

4.1. CLASS CARTILAGINOUS FISHES, CHONDRICHTHYES

4.1.3. Origin

Cartilaginous Fishes appeared on Earth long before the dinosaurs. The earliest fossil finds date back 220–265 million years. At that time, the ancestors of modern sharks already possessed a fairly advanced level of Organization. The oldest fossil sharks have been found in numerous marine limestone

deposits in Ohio (USA). Several Teeth similar to those of modern sharks were discovered there. Thanks to another discovery in Cleveland—well-preserved remains of the body and some Internal Organs—scientists have reconstructed the primitive shark. These specimens ranged in size from 45 to 120 cm.

The ancestors of modern sharks went extinct during the Permian period, giving way to the further Development of the hybodont—an animal resembling modern sharks that had sharp teeth at the front of its jaws and broad teeth at the back of its Mouth, adapted for crushing mollusk shells. Hybodonts lived in constant competition with the ancestors of bony fishes and carnivorous reptiles. Gradually, hybodonts began to give way to new shark species, one of which—the bullhead shark—still lives off the coast of Australia.

During the Jurassic period, the age of the dinosaurs, sharks evolved successfully. Many families arose during this time, including the ancestors of modern rays. By the late Miocene, sharks were among the most common animals inhabiting the seas. Another branch of ancient cartilaginous fish gave rise to the classes of placoderms and bony fishes.