Sharks

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Sharks
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 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 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 fins 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 ...

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