Evolution And Classification
Sharks are often described as "primitive" animals, and little changed in millions of years of evolution. It is true that the first sharks evolved in the oceans more than 300 million years ago, in the Devonian era. However, the earliest species of sharks are all extinct. The species living in the oceans today evolved only 70-100 million years ago. The fact that the general body plan of the earliest sharks was so similar to that of living ones is a testimony to the suitability of their adaptation to the environment in which sharks still live.
Sharks and other modern fish are descended from primitive fish, called Placoderms, that were covered with bony, armor-like plates. The descendants of the Placoderms lost the armor, but retained an internal skeleton. Most types of modern fish, such as trout, minnows, and tuna, have a bony skeleton. Sharks and their relatives, the skates and rays, are distinguished from other types of fish in that they have cartilage rather than bone as their skeletal material (cartilage is a translucent, flexible, but strong material that also makes up the ears and nose of mammals, including humans). Thus, the sharks are called the "cartilaginous fishes" (class Chondrichthyes).