Unraveling the nature of the whorl-tooth

Unraveling the nature of the whorl-toothed shark
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Reconstructing the anatomy of
sharks isn't easy. With few exceptions -- an
exquisitely-preserved body fossil here, some calcified bits of
skeleton there -- teeth make up the majority of the
fossil record. When those teeth come from a relatively
recent species with close living relatives, it is not difficult to
imagine what the extinct species might have looked like. The
further back in time you go, though, the more bizarre sharks
become. Sometimes teeth are not enough, and one especially unusual
set of teeth has vexed paleontologists for over a century.
At first sight, the teeth didn't look like they belonged to a
shark at all. Coiled upon itself in a circular whorl, the bizarre

row superficially resembled the shells of extinct cousins
of nautilus and squid called ammonites. After studying
tooth whorls found in the Ural Mountains, though, the Russian
geologist Alexander Petrovich
recognised them for what they were. In 1899 he
described them under the name Helicoprion as the remains
of an ancient shark. Just how they fit in the shark's mouth was
another matter altogether.
No known shark had a similar buzz-saw arrangement of teeth, and,
although an exact date for it could not be pinned down,
Helicoprion had clearly lived long before the appearance
of modern sharks. (Today we know that Helicoprion was a
widespread genus of shark that persisted from about 290 to 270
million years ago.) Karpinsky was on his own, and in his original
restorations he placed the tooth whorl on the shark's snout with
the teeth jutting from the outer edge. Not everyone agreed. While
praising Karpinsky's diligent description, in 1900 the
paleontologist Charles Rochester Eastman balked at the restoration,
writing "Few will be prepared to admit, however, that this highly
fanciful sketch can be taken seriously, and, therefore, the least
said about it the better." Drawing a comparison with similar
of a shark called Edestus, Eastman pointed out
that some authorities believed such tooth-like structures were
actually spines embedded on the back of the sharks. Which
restoration was correct seemed impossible to say.
Karpinsky did not stick to his original idea. Taking the
criticism of his peers seriously, he restored the whorls elsewhere
on the shark's body -- on the dorsal fin, sticking out of the back,
or extending from the upper tip of the tail. After all, some living
sharks had spines or spikes on their bodies, and shark skin itself
is made up of tiny, tooth-like structures called dermal
, so it was not out of the question that the objects
might have been used for defense or ornamentation.

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