It sounds like something out of a Hollywood movie script, but it really happened: Shark evolution researchers say increased ocean temperatures more than 100 million years ago may have caused sharks to get bigger, swim faster and become the powerful predators we know today.
In a paper published last month in the journal Current Biology, the scientists reported that they measured the fin sizes and body lengths of 500 extinct and living sharks and compared that information with data from the shark evolutionary family tree. Their results suggest that when the ocean became too warm about 122 million years ago during the Cretaceous period, some sharks abandoned their seafloor environment and moved up into the open ocean. This ascent may have changed their fin and body structure, which led to changes in their size and ability to swim.
It’s a misconception that all sharks are like the bloodthirsty, powerful and streamlined beasts of “Jaws” 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 are bottom feeders. Unlike their pelagic – or open water – relatives, demersal sharks do not need to constantly swim to breathe. They can rest at the bottom of the sea.
The need to breathe, however, may have been just the urge that drove some sharks higher in the water column. Cretaceous ocean floors, the authors argue, may have become increasingly oxygen-poor in places. In order for the ancestors of many modern sharks to survive and eventually thrive, it was time for them to burrow into the seabed.
Evidence of this habitat change, and what endured in which environment, can be seen in the changing pectoral fins of ancient pelagic and benthic sharks.
“Most of the open-water sharks tend to have elongated fins, and benthic sharks have more persistent fins,” said Lars Schmitz, a biology professor at Claremont McKenna College in California, who is an author of the paper.
Fellow author Phillip Sternes, a California-based shark researcher, compared the pectoral fins to the wings of an airplane. “Long, narrow wings” — like those of a commercial airplane, for example — “help the lift-to-drag ratio, so it lowers fuel costs,” he said. In contrast, the “short, stubby wings of fighter jets are not good for long-distance travel, but they can cost a penny.”
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 wings that may have increased. The Cretaceous ocean surface temperature peak of 83 degrees Fahrenheit may have affected the shark’s speed. (For comparison, today’s average is 68 degrees.)
Sharks and other fish are similar to most animals, explained co-author Timothy Higham, a professor at the University of California, Riverside, “in that muscle function is very temperature dependent.” In other words, he said, “if your muscles warm up, they get better at contracting quickly.”
Warmer temperatures and faster, faster muscles meant the sharks “could flick their tail back and forth faster,” he said. This translates into increased speed, which, he added, may have then led the sharks to “expand into a more open water habitat,” catching fast-swimming prey and avoiding other Cretaceous marine predators that are now extinct.
That all sounds advantageous. With ocean temperatures now 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 to some sharks, but Dr. Higham stressed that today’s rapidly changing climate is more likely to lead to damage to ocean life.
“Because other animals, organisms that aren’t sharks, were completely destroyed,” 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 Cal State University, Humboldt, who was not involved in the research, agreed.
“The spread of marine anoxic zones and changes in global climate, often accompanied by ocean acidification, have led to the worst mass extinctions in Earth’s history,” he said, adding that “the pace of change now is truly unprecedented.” .