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Teeth and jaws
The most famous assets to the sharks is probably their
teeth. If you mention the word shark, people usually think of a huge
mouth with big, white, sharp teeth. The rumor may be exaggerated,
but still, there is something special about the jaws and teeth of
a shark.
 
Fossil shark teeth (left) and teeth from the porbeagle,
Lamna nasus(right).
(© J. Lokrantz/Azoteimages)
A shark tooth can be big or small, sharp or blunt, serrated or smooth.
The shape of the teeth is an adaptation to the feed the animal eat.
Flat teeth are useful to crush mollusks and crustaceans, while sharp
teeth are more adapted to rip chunks of flesh out of a prey. One thing
that all different types of teeth have in common is that they are
exchangeable throughout the entire life. The teeth are attached to
multiple rows in the jaws. Behind each tooth there are several new
ones waiting to be used. Some sharks change their teeth randomly while
others change the entire row at the same time. The teeth are embedded
in the gums but not attached directly to the jawbone, as in other
fishes.
(© J. Lokrantz/Azoteimages)
The teeth are actually enlarged and modified placoid scales and just
like our teeth, they consists of an outer enamel layer and an inner
pulp. Most sharks only use the first or the two rows of teeth in the
front, but some species use as much as eight rows at the same time.
The rows that are not in use lie with the teeth bent back in the gums
and wait for their turn. Each new tooth is slighter larger than the
tooth it replaces. The older the shark gets and the more it grows,
the larger its teeth gets. The size of the teeth of the same individual
can vary depending on where in the jaw the tooth is. Some species
have however all teeth in the same size.
The exchange of teeth goes on the entire life and for some species
it has been estimated that they use as many as 30,000 teeth during
their lifetime. The jaws are not firmly fixed to the braincase, but
instead loosely connected. This loose attachment allows sharks to
widen their mouth and extend the upper jaw so they can engulf large
items of food and even whole animals.
Ref: Hickman CP, Roberts LS and Larson
A (2000), Animal Diversity, McGraw-Hill Higher Education, USA -- Springer
VG and Gold JP (1989), Sharks in quetion: the Smithsonian answear
book, Smithsonian Institution Press, Washington, D.C.
London
 Further reading
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