Sharks :: Papers

INTRODUCTION Although sharks belong to the class Chondrichtyes, there are many different types. Sharks arose about 350 million years ago and have remained virtually unchanged for the past 70 million years and still comprise a dominant group. It is thought that sharks almost certainly evolved from placoderms, a group of primitive jawed fishes. It took a long series of successful and unsuccessful mutations with fin, jaw positions etc to give us all the different designs of sharks around today. When asked to draw a shark, most people would draw a shape along the lines of the whaler shark family, tigers or a mackeral shark such as a porbeagle. However many people do not realize the sheer diversity in the shape of sharks, or that rays are really sharks. Seldom does such an animal inspire such a variety of emotions reflecting a mixture of fascination, awe and fear. Sharks have occasionally exacted a terrible price from humans who have trespassed on their territory. No better understood than the ocean t hat they inhabit, these creatures should be regarded in the same way as lions, tigers, and bears: as dangerous, predatory but nonetheless magnificent animals. Different Types of Sharks Living sharks are divided into eight major orders, each easily recognizable by certain external characteristics. Each order contains one or more smaller groups, or families. In all there are 30 families of sharks and they contain the 350 or more different kinds or species of sharks. The eight major orders of sharks include the Squantiformes, Pristiophormes, Squaliformes, Hexanchiformes, Carcharhiniformes, Lamniformes, Orectolobiformes, and the Heterodotiformes. The orders have distinguishing characteristics that fit in each. The Squantiformes normally have flat bodies that are ray-like with mottled dorsal surfaces. These sharks have a short terminal mouth, which is armed with small impaling teeth. They also have a caudal fin, which has a lower lobe that is longer than the upper lobe. Their pectoral fi ns extend forward over the ventrally directed gills. The Pristiophormes have more of an elongated snout, which is saw-like and edged with slender, needle-sharp lateral teeth. They have two dorsal fins and no anal fin. They use short transverse mouths and small cuspidate holding teeth in both jaws. Squaliformes have no anal fin as well, but their snout is not elongated, but is somewhat long. Many have powerful cutting teeth in both jaws. In some species these razor sharp teeth are in the lower jaw only and the upper teeth serve to hold the food.