The sharks we know today as the open ocean's top predators evolved from stubby bottom dwellers during a dramatic episode of global warming millions of years ago.
"The pectoral fins are a critical structure, comparable to our arms," said UCR biology doctoral student and paper first author Phillip Sternes."What we saw upon review of a massive data set, was that these fins changed shape as sharks expanded their habitat from the bottom to the open ocean." "The data helped us make a correlation between higher temperatures, tail movement, and swimming speeds," Higham said.
Modern sea surface temperatures average about 68 degrees Fahrenheit. In the Cretaceous they were much warmer, reaching an average of about 83 degrees. The high heat of the Cretaceous did not happen overnight, and neither did the sharks' evolution. Biologists are seeing some sharks, including tropical species like tiger and bull sharks, starting to swim farther north. But it is unclear whether threatened sharks will again be able to adapt where they live and survive the rapidly increasing heat.
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