It sounds like something out of a Hollywood movie script, but it really happened: Researchers who study shark evolution say warmer ocean temperatures more than 100 million years ago may have allowed sharks to become the larger, faster swimmers and more powerful predators they are known as today.
In a paper published last month in the journal Current Biology, scientists reported that they measured the fin size and body length of 500 extinct and modern shark species and compared that information with data from the shark evolutionary tree. They found that when the oceans became very warm about 122 million years ago during the Cretaceous period, some sharks abandoned their undersea habitats and migrated into the open ocean. This upswing may have altered the structure of their fins and bodies, altering their size and swimming ability.
It's a misconception that all sharks are like the powerful, bloodthirsty, streamlined beasts from the movie “Jaws” that swim near the ocean's surface (or through tornadoes and city streets, if you've seen “Sharknado”). Most sharks are traditionally benthic, or bottom-dwelling animals. Unlike their pelagic (i.e. open-ocean) cousins, benthic sharks don't need to constantly swim to breathe; they can rest on the ocean floor.
But the need to breathe may simply have prompted some sharks to move higher in the water column: The Cretaceous ocean floor may have become increasingly oxygen-poor in places, the authors speculate, and it was time for the ancestors of many modern sharks to abandon the ocean floor in order to survive and ultimately thrive.
Clues to this habitat shift and what survived in what environment can be found in changes to the pectoral fins of ancient pelagic and benthic sharks.
“Most ocean-dwelling sharks tend to have long fins, while bottom-dwelling sharks have short fins,” said study author Lars Schmitz, a professor of biology at Claremont McKenna College in California.
Co-author Philip Stearns, a California-based shark researcher, likened pectoral fins to airplane wings. “Long, slender wings, like those of a commercial airliner, increase the lift-to-drag ratio and reduce fuel costs,” Stearns said. In contrast, “the short, stubby wings of a fighter jet are not ideal for long-distance flight, but they can change direction quickly.”
The same is true for sharks: longer pectoral fins may have allowed larger sharks to swim more efficiently, an important adaptation for species that needed to constantly swim to breathe.
But it may not just be body and fin size that increased: Ocean surface temperatures during the Cretaceous period peaked at about 83 degrees Fahrenheit, which could have affected the sharks' speed. (For comparison, today's average is 68 degrees Fahrenheit.)
Sharks and other fish are like most animals in that their muscle function depends heavily on temperature, explained co-author Timothy Higham, a professor at the University of California, Riverside. In other words, “as your muscles warm up, they can contract faster.”
Rising temperatures and faster, more agile muscles allowed sharks to “swing their tails back and forth faster,” he said. That increased speed could have allowed sharks to “expand into larger water habitats,” catch faster-swimming prey and avoid now-extinct Cretaceous marine predators, he added.
It all sounds advantageous. Now that global warming is causing ocean temperatures to rise, could modern sharks see similar changes? Could they become bigger and faster?
Global warming millions of years ago may have led to important evolutionary adaptations for some sharks, but Dr Higham stressed that today's rapidly changing climate is likely to take a toll on marine life.
“Everything else was devastated,” he said, adding that while some sharks had adapted to the Cretaceous oceans, “a lot of other animals went extinct too.”
Alison Bronson, a faculty member at California Polytechnic State University, Humboldt, who was not involved in the study, agreed.
“The expansion of oxygen-free zones in the oceans and changes in the Earth's climate are often occurring simultaneously with ocean acidification, causing the worst mass extinction event in Earth's history,” she said, adding that “the speed of the current changes is truly unprecedented.”
