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The researchers looked at carbon and oxygen isotopes in multiple species of planktonic and benthic foraminifera.

Instead, anglerfish achieved high levels of phenotypic disparity, greater than their benthic relatives in both shallow and deep waters.

Most living shark species are still bottom dwellers, occupying what scientists refer to as the benthic zone.

“Urchins are critical components of benthic ecosystems, and there are pretty tight tipping points: We don’t want too many or too few,” Harvell says.

Their analyses show that the rapid transition of ancestrally bottom-dwelling, or benthic, anglerfishes into open-ocean, or pelagic, habitats occurred during a period of major global warming 50 to 35 million years ago.