It sounds like something out of a Hollywood movie script, but it really happened: Shark evolution researchers say that increased ocean temperatures more than 100 million years ago may have caused sharks to grow larger, swim faster and become powerful predators like we know today.
In a paper published last month in the journal Current Biology, researchers said they measured the fin sizes and body lengths of 500 extinct and living sharks and compared that information with data from the sharks’ evolutionary family tree. Their results suggest that when the ocean became very warm during the Cretaceous period around 122 million years ago, some sharks left their habitats on the sea floor and moved into the open ocean. This ascent may have altered their fin and body structure, leading to changes in their size and swimming ability.
It’s a misconception that all sharks are like the bloodthirsty, powerful, aerodynamic “Jaws” beasts that swim near the surface of the ocean (or in tornadoes and city streets, if you’ve watched “Sharknado”). Most sharks have always been benthic, meaning they feed on the bottom. Unlike their pelagic – or free-living – relatives, demersal sharks do not need to swim constantly to breathe. They can rest on the sea floor.
However, the need to breathe may have been just the incentive that pushed some sharks higher in the water column. The bottoms of the Cretaceous oceans, the authors assume, may be increasingly poor in oxygen in places. In order for the ancestors of many modern sharks to survive and eventually thrive, it was time to leave the sea floor.
Clues to this habitat shift and what survived in what environment can be seen in the changing pectoral fins of ancient pelagic and benthic sharks.
“Most open-water sharks tend to have elongated fins, and benthic sharks have thicker fins,” said Lars Schmitz, a professor of biology at Claremont McKenna College in California, who authored the paper.
His colleague Phillip Sternes, a shark researcher based in California, compared the pectoral fins to the wings of an airplane. “Long, narrow wings” — such as those of a commercial airplane — “help your lift-to-drag ratio, so they lower your fuel costs,” he said. In contrast, “the short, thick wings of fighters are not suitable for long-distance travel, but they can turn on a dime”.
The same is true for sharks: Longer pectoral fins may have made swimming more efficient for larger-bodied sharks, an important adaptation for species whose breathing now required constant swimming.
But it is not only the size of the body and fins that need to increase. Peak Cretaceous ocean surface temperatures of about 83 degrees Fahrenheit may have affected the sharks’ speed. (For comparison, today’s average is 68 degrees.)
Timothy Higham, co-author and professor at the University of California, Riverside, explained that sharks and other fish are similar to most animals, “in that muscle function is very temperature dependent. In other words, he said, “If your muscles warm up, they’ll contract better quickly.”
Warmer temperatures and faster, faster muscles meant the sharks “could whip their tail back and forth faster,” he said. This translates into increased speed, which, he added, may have then led to the sharks “expanding into the open water environment,” catching fast-swimming prey and avoiding other Cretaceous marine predators that are now extinct.
Which all sounds advantageous. With ocean temperatures rising due to global warming, could we see similar changes in today’s sharks? In other words, could sharks get even bigger and faster?
Global warming millions of years ago may have introduced important evolutionary adaptations in some sharks, but Dr. Higham emphasized that today’s rapidly changing climate is more likely to lead to damage to ocean life.
“Because the other animals, the non-shark organisms, were completely devastated,” he said. He added that while some sharks adapted to the Cretaceous oceans, “it also caused the extinction of many other animals.”
Allison Bronson, a faculty member at California State Polytechnic University, Humboldt, who was not involved in the research, agreed.
“The expansion of marine anoxic zones and global climate change, often occurring together with ocean acidification, have led to the worst mass extinction in Earth’s history,” she said, adding that “the pace of change is now truly unprecedented.”