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JOHN MAISEY>> The fossil record is dominated
by shark’s teeth.
Shark’s teeth are among one of the most
common vertebrate fossils that you can find,
but the skeletons are exceptionally rare.
Sharks and their relatives don’t have lots
of bones covering the head and the body like
a fish you buy at the supermarket.
The most bony things are, in fact, the teeth.
They’re made of dentine and enamel-like
tissue, just like our teeth,
but the rest of the skeleton is just soft cartilage
coated with this hard calcium phosphate layer.
It’s not bone, but it consist of literally
hundreds of thousands of little, tiny crystals
or fragments of calcite that are held together
by collagen fibers.
And they all gradually get bigger and bigger
as the fish grows,
but they’re not solid bone.
So one of the things about a shark skeleton
is after the shark dies,
very often when the collagen fibers disintegrate—because they’re organic and they decompose—
the skeleton just falls apart.
And that’s not good for fossils.
Because the fossils often just break up as a result.
The only time you ever find really
complete fossils of shark-like fishes
is when they’ve been buried very rapidly in
the sediment and the mud
and they’ve been removed from any scavengers
and they haven’t decomposed
and things don’t get moved around by currents and so forth.
There are no solid bones to hold together,
so the whole thing is very vulnerable to just
collapsing and falling apart.
So when somebody does find a fossil shark–and
especially one with a skeleton that’s preserved
in three dimensions–it’s a really big
deal because they are so rare and so fragile.
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