How the shark forgot his skeleton

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How the shark forgot his skeleton
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The discovery of a 410m-year-old fossil in Mongolia has caused palaeontologists to rethink a theory on skeletons

animals with a backbone—are a diverse bunch, encompassing everything from tuna and budgerigars to snakes, chinchillas and human beings. One way biologists divide them up concerns the composition of their skeletons. Most vertebrates sport hard, calcified bones, and are dubbed the osteichthyans. A second, much smaller category is the chondrichthyans, whose members include sharks, rays and skate.

Palaeontologists had long assumed that cartilage was the more primitive arrangement. Osteichthyan fetuses, after all, begin life with a cartilaginous skeleton that is gradually replaced by harder, more durable bone as they grow. But a paper just published insuggests that view may be mistaken. In it a team led by Martin Brazeau of Imperial College London describe the discovery of a 410m-year-old fossil in Turgen, a district of Mongolia close to the Russian border.

Exactly why is a matter of conjecture. One possibility is that it was an evolutionary adaptation. “Sharks don’t have swim bladders, which evolved later in bony fish, but a lighter skeleton would have helped them be more mobile in the water and swim at different depths,” speculates Dr Brazeau. “This may be what helped sharks to be one of the first global fish species, spreading out into oceans around the world 400m years ago.

Whatever the reason, the fact that sharks are still around 400m years later suggests that a soft skeleton is a highly successful strategy, evolutionarily speaking. Yet it may also be a limited one. The swim bladders developed by bony fish to modulate their buoyancy would soon be co-opted for another purpose: transformed into a primitive set of lungs, they allowed their bearers to breathe air, and therefore to colonise the land.

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