Modern-type crow shark

Sharks swam in seas and rivers during the Carboniferous. Although they had evolved millions of years earlier, more distinct types lived in the Carboniferous than at any other time in Earth history.

Why So Successful?
Scientists think that two things helped sharks explode in diversity: few competitors for food and the absence of predators. Placoderms, which ate sharks and competed against them, went extinct around 345 million years ago—making life much easier for sharks.

Sharks Today
Modern shark lineages didn’t arise until the Cretaceous (145 to 100 million years ago). Although sharks are sometimes called "“living fossils,"” the sharks we know today have evolved a lot since their ancestors swam Carboniferous seas.
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Shark skeletons are made of cartilage—the material that stiffens our noses—rather than bone.