Sharks :: Papers

INTRODUCTIONAlthough sharks belong to the class Chondrichtyes, there ar many different types. Sharks arose about 350 million years ago and drive remained virtually unchanged for the past 70 million years and thus far 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 self-defeating mutations with fin, jaw positions etc to give us all the different designs of sharks around today. When asked to drop behind 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 brighten the sheer diversity in the shape of sharks, or that rays ar really sharks. Seldom does such an animal inspire such a bod of emotions reflecting a mixture of fascination, awe and fear. Sharks have occasionally exacted a terrific price from humans who have trespassed on their territory. No bette r mum than the ocean that 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 study orders, each easily recognizable by certain external characteristics. Each order contains one or more(prenominal) 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 categorical bodies that are ray-like with mottled dorsal surfaces. These sharks have a myopic close mouth, which is armed with small impaling teeth. They also have a caudal fin, which has a lower lobe that is l onger than the hurrying lobe. Their pectoral fins extend forward over the ventrally directed gills. The Pristiophormes have more of an elongated soapbox, which is saw-like and edged with slender, needle- cracking squint-eyed teeth. They have two dorsal fins and no anal fin. They use short transverse mouths and small cuspidate holding teeth in twain jaws. Squaliformes have no anal fin as well, but their snout is not elongated, but is somewhat long. Many have powerful cut of meat 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.