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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 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 jaw only and the upper teeth serve to hold the
food. Hexanchiformes have six or seven gill slits. They are sharks with a single
spineless dorsal fin, and an anal fin. The typical Carcharhiniforme has an
elongated snout, a long mouth that reaches behind the eyes, an anal fin and two
spineless dorsal fins.
The eyes have movable, nictitating lower eyelids worked by unique muscles.
Teeth vary from small and cuspidate or flattened to large and bladelike.
Carcharhiniformes have no enlarged rear crushing teeth. Along with this they
have a spiral scroll intestinal valve. A Lamniforme shark has an elongated
snout. Most have long mouths that reach behind the eyes, an anal fin and two
spineless dorsal fins.
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